Use Github Issue Templates

[codebrainz]

We should use the improved version of Github Issue Templates as recommended by @andy5995 in #2063.

I have created a sample repository using almost the stock issue templates for bug reports and feature requests, except I adapted the version information section in the bug report template for our use.

Please feel free to create issues there to test what it's like with the templates.

Concretely, this means adding a .github/ISSUE_TEMPLATE directory and then a Markdown file for each issue template in that directory. Github has some kind of web-based issue template editor/previewer, which is what I used to create the two templates using almost their default contents.

References:
https://help.github.com/en/articles/about-issue-and-pull-request-templates
https://help.github.com/en/articles/creating-issue-templates-for-your-repository

Supersedes #1647 and #2063.

[eht16]

LGTM.
We probably can add another one for pull requests (based on the one for feature requests) and additionally mention tests (manual testing, different GTK versions, make distcheck, ...).

[elextr]

@eht16 we need to be careful what we add.

Whilst we want to encourage more complete pull requests, since we don't have surplus effort to do things ourselves more contributor effort is always appreciated, but we should not make it look like contributors must to do all these other things or that the pull request will be rejected if they are not done.

Also some of those really only apply to code changes, not to documents or configurations etc.

Since pull request contributors are more invested we can afford to make the pull request template a little more extensive than issue/feature requests, but it should be presented in a way that it is clear that it can be ignored if the contributor can't do it, or doesn't think its relevant.

[elextr]

I should add that the templates LGTM also. Only suggestion would be to move the version info section of the bug report up a bit to (subtly :) indicate its important.