* FIX: Improve GitHub folder regexp in Onebox
It used to match any GitHub URL that was not matched by the other GitHub
Oneboxes and it did not do a good job at handling those. With this
change, the generic Onebox will handle the remaining URLs.
* FEATURE: Add Onebox for GitHub Actions
* FEATURE: Add Onebox for PR check runs
* FIX: Remove image from GitHub folder Oneboxes
It is a generic, auto-generated image which does not provide any value.
* DEV: Add tests
* FIX: Strip HTML comments from PR body
