GitHub Flow migration (#14)

Adapted to "GitHub Flow" [1] like documented in detail in the main task
issue svengreb/styleguide-git#9 [2].

[1]: https://guides.github.com/introduction/flow
[2]: svengreb/styleguide-git#9

Co-authored-by: Sven Greb <development@svengreb.de>

Subtask of svengreb/styleguide-git#9
GH-13

CHANGELOG.md

@@ -48,15 +48,15 @@ Please see the [corresponding issue in the Nord Docs repository][nord-docs#175]
 
 ## Features
 
-Detailed information about features and install instructions can be found in the [README](https://github.com/arcticicestudio/nord-slack/blob/develop/README.md#installation) and in the [project wiki](https://github.com/arcticicestudio/nord-slack/wiki).
+Detailed information about features and install instructions can be found in the [README](https://github.com/arcticicestudio/nord-slack/blob/master/README.md#installation) and in the [project wiki](https://github.com/arcticicestudio/nord-slack/wiki).
 
-<p align="center"><img src="https://raw.githubusercontent.com/arcticicestudio/nord-slack/develop/assets/scrot-hero.png"/></p>
+<p align="center"><img src="https://raw.githubusercontent.com/arcticicestudio/nord-slack/master/assets/scrot-hero.png"/></p>
 
-<p align="center"><strong>Menu- and item hovering</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-slack/develop/assets/scrcast-hovering.gif"/></p>
+<p align="center"><strong>Menu- and item hovering</strong><br><img src="https://raw.githubusercontent.com/arcticicestudio/nord-slack/master/assets/scrcast-hovering.gif"/></p>
 
 ❯ Defined the theme colors according to the [Nord Style Guidelines][gh-repo]. (@arcticicestudio, #1 in PR #2, b37cbde4)
 
-<p align="center"><img src="https://raw.githubusercontent.com/arcticicestudio/nord-slack/develop/assets/scrot-definitions.png"/></p>
+<p align="center"><img src="https://raw.githubusercontent.com/arcticicestudio/nord-slack/master/assets/scrot-definitions.png"/></p>
 
 <!--
 +------------------+

README.md

@@ -1,6 +1,6 @@
 <p align="center"><a href="https://www.nordtheme.com/ports/slack" target="_blank"><img src="https://raw.githubusercontent.com/arcticicestudio/nord-docs/develop/assets/images/ports/slack/repository-hero.svg?sanitize=true"/></a></p>
 
-<p align="center"><a href="https://github.com/arcticicestudio/nord-slack/releases/latest" target="_blank"><img src="https://img.shields.io/github/release/arcticicestudio/nord-slack.svg?style=flat-square&label=Release&logo=github&logoColor=eceff4&colorA=4c566a&colorB=88c0d0"/></a> <a href="https://www.nordtheme.com/docs/ports/slack" target="_blank"><img src="https://img.shields.io/github/release/arcticicestudio/nord-slack.svg?style=flat-square&label=Docs&colorA=4c566a&colorB=88c0d0&logo=data%3Aimage%2Fsvg%2Bxml%3Bbase64%2CPHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxNiIgaGVpZ2h0PSIxNiI%2BCiAgICA8cGF0aCBmaWxsPSIjZDhkZWU5IiBkPSJNMTMuNzQ2IDIuODEzYS42Ny42NyAwIDAgMC0uNTU5LS4xMzNMOCAzLjg0OGwtNS4xODgtMS4xOGEuNjY5LjY2OSAwIDAgMC0uNTcuMTMzLjY3Ny42NzcgMCAwIDAtLjI0Mi41MzF2OC4xMzNjLS4wMDguMzIuMjEuNTk4LjUyLjY2OGw1LjMzMiAxLjE5OWguMjk2bDUuMzMyLTEuMmEuNjY4LjY2OCAwIDAgMCAuNTItLjY2N1YzLjMzMmEuNjU5LjY1OSAwIDAgMC0uMjU0LS41MnpNMy4zMzIgNC4xNjhsNCAuODk4djYuNzY2bC00LS44OTh6bTkuMzM2IDYuNzY2bC00IC44OThWNS4wNjZsNC0uODk4em0wIDAiLz4KPC9zdmc%2BCg%3D%3D"/></a> <a href="https://github.com/arcticicestudio/nord-slack/blob/develop/CHANGELOG.md" target="_blank"><img src="https://img.shields.io/github/release/arcticicestudio/nord-slack.svg?style=flat-square&label=Changelog&logo=github&logoColor=eceff4&colorA=4c566a&colorB=88c0d0"/></a></p>
+<p align="center"><a href="https://github.com/arcticicestudio/nord-slack/releases/latest" target="_blank"><img src="https://img.shields.io/github/release/arcticicestudio/nord-slack.svg?style=flat-square&label=Release&logo=github&logoColor=eceff4&colorA=4c566a&colorB=88c0d0"/></a> <a href="https://www.nordtheme.com/docs/ports/slack" target="_blank"><img src="https://img.shields.io/github/release/arcticicestudio/nord-slack.svg?style=flat-square&label=Docs&colorA=4c566a&colorB=88c0d0&logo=data%3Aimage%2Fsvg%2Bxml%3Bbase64%2CPHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIxNiIgaGVpZ2h0PSIxNiI%2BCiAgICA8cGF0aCBmaWxsPSIjZDhkZWU5IiBkPSJNMTMuNzQ2IDIuODEzYS42Ny42NyAwIDAgMC0uNTU5LS4xMzNMOCAzLjg0OGwtNS4xODgtMS4xOGEuNjY5LjY2OSAwIDAgMC0uNTcuMTMzLjY3Ny42NzcgMCAwIDAtLjI0Mi41MzF2OC4xMzNjLS4wMDguMzIuMjEuNTk4LjUyLjY2OGw1LjMzMiAxLjE5OWguMjk2bDUuMzMyLTEuMmEuNjY4LjY2OCAwIDAgMCAuNTItLjY2N1YzLjMzMmEuNjU5LjY1OSAwIDAgMC0uMjU0LS41MnpNMy4zMzIgNC4xNjhsNCAuODk4djYuNzY2bC00LS44OTh6bTkuMzM2IDYuNzY2bC00IC44OThWNS4wNjZsNC0uODk4em0wIDAiLz4KPC9zdmc%2BCg%3D%3D"/></a> <a href="https://github.com/arcticicestudio/nord-slack/blob/master/CHANGELOG.md" target="_blank"><img src="https://img.shields.io/github/release/arcticicestudio/nord-slack.svg?style=flat-square&label=Changelog&logo=github&logoColor=eceff4&colorA=4c566a&colorB=88c0d0"/></a></p>
 
 <p align="center"><a href="https://github.com/arcticicestudio/styleguide-markdown/releases/latest" target="_blank"><img src="https://img.shields.io/github/release/arcticicestudio/styleguide-markdown.svg?style=flat-square&label=Markdown%20Style%20Guide&colorA=4c566a&colorB=88c0d0&logo=data%3Aimage%2Fsvg%2Bxml%3Bbase64%2CPHN2ZyB4bWxucz0iaHR0cDovL3d3dy53My5vcmcvMjAwMC9zdmciIHdpZHRoPSIzOSIgaGVpZ2h0PSIzOSIgdmlld0JveD0iMCAwIDM5IDM5Ij48cGF0aCBmaWxsPSJub25lIiBzdHJva2U9IiNEOERFRTkiIHN0cm9rZS13aWR0aD0iMyIgc3Ryb2tlLW1pdGVybGltaXQ9IjEwIiBkPSJNMS41IDEuNWgzNnYzNmgtMzZ6Ii8%2BPHBhdGggZmlsbD0iI0Q4REVFOSIgZD0iTTIwLjY4MyAyNS42NTVsNS44NzItMTMuNDhoLjU2Nmw1Ljg3MyAxMy40OGgtMS45OTZsLTQuMTU5LTEwLjA1Ni00LjE2MSAxMC4wNTZoLTEuOTk1em0tMi42OTYgMGwtMTMuNDgtNS44NzJ2LS41NjZsMTMuNDgtNS44NzJ2MS45OTVMNy45MzEgMTkuNWwxMC4wNTYgNC4xNnoiLz48L3N2Zz4%3D"/></a> <a href="https://github.com/arcticicestudio/styleguide-git/releases/latest" target="_blank"><img src="https://img.shields.io/github/release/arcticicestudio/styleguide-git.svg?style=flat-square&label=Git%20Style%20Guide&logoColor=eceff4&colorA=4c566a&colorB=88c0d0&logo=git"/></a></p>
 
@@ -53,7 +53,7 @@ The guide also includes information about [minimal, complete, and verifiable exa
 
 <p align="center">Copyright &copy; 2017-present <a href="https://www.arcticicestudio.com" target="_blank">Arctic Ice Studio</a> and <a href="https://www.svengreb.de" target="_blank">Sven Greb</a></p>
 
-<p align="center"><a href="https://github.com/arcticicestudio/nord-slack/blob/develop/LICENSE.md"><img src="https://img.shields.io/static/v1.svg?style=flat-square&label=License&message=MIT&logoColor=eceff4&logo=github&colorA=4c566a&colorB=88c0d0"/></a></p>
+<p align="center"><a href="https://github.com/arcticicestudio/nord-slack/blob/master/LICENSE.md"><img src="https://img.shields.io/static/v1.svg?style=flat-square&label=License&message=MIT&logoColor=eceff4&logo=github&colorA=4c566a&colorB=88c0d0"/></a></p>
 
 [nord-comm]: https://www.nordtheme.com/community
 [nord-contrib-guide-branching]: https://github.com/arcticicestudio/nord/blob/develop/CONTRIBUTING.md#branch-organization