Replace markdown implementation with thephpleague/commonmark

[glensc]

thephpleague/commonmark seems better library, when considering extensibility.

for example:

• Add Github like mentions autocomplete #250 - https://commonmark.thephpleague.com/customization/inline-parsing/#example-1---twitter-handles
• emojis - https://commonmark.thephpleague.com/customization/inline-parsing/#example-2---emoticons

refs:

• Fix parsing of link references in markdown #367
• Update cebe/markdown to 1.2.1 #365
• Add experimental markdown rendering #291
• Add TaskListMatcher with example to update issue complete percentage #473
• markdown: make table render nicer #465

---

• basic markdown rendering
• strike through: Strikethrough support thephpleague/commonmark-extras#6, https://github.com/uafrica/commonmark-ext
• autolink: Autolink support without <...> thephpleague/commonmark-extras#12 via https://github.com/thephpleague/commonmark-ext-autolink
• enable markdown for all users (db patch)
• tables: https://github.com/webuni/commonmark-table-extension
• inline render to avoid <p></p> in html: https://github.com/thephpleague/commonmark-ext-inlines-only
• (optional) emojis: https://github.com/AltThree/Emoji
• (optional) A sup/sub extension: https://github.com/OWS/commonmark-sup-sub-extensions
• tasklist: https://github.com/lossendae/commonmark-task-lists
• (optional) improve table look and feel: markdown: make table render nicer #465
• (optional) front matter: https://docs.gitlab.com/ce/user/markdown.html#front-matter

fixes: #465

[glensc]

this is complete. the optional parts can be done later.