Small fixes and improvements for GH comments and related

[Urgau]

This PR is a bag of small fixes and improvements for our GH comments and related endpoint, specifically it:

1. Add license information for our embedded copy github-markdown-css
2. Extract GitHubComments cache logic into a re-usable cache and make us of it for gh-comments and gha-logs
   • As part of this, I also fixed the pruning logic to remove the value size
3. Add alias /issues for our GH Comment endpoint to simplify access from GitHub url
4. Add a message about pull-requests being not yet supported
5. Regenerate our copy of GitHub's Markdown css (as we were missing ~1yr worth of updates)
6. Allow inline style attributes to fix CSP violations

Best reviewed commit by commit.

[Kobzol]

Thank you!

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