Gitea / Forgejo issue integration

Hello Outline team,

we recently migrated off GitHub to our own [Forgejo]( instance, both for managing code and as an issue-tracking / project management platform. For that, an outline integration would be nice similarly to #6406 #6795 or #4427 for seeing things like issue status.

I would also offer to take a shot at this, was thinking it should not differ too much from the GitLab integration. Nevertheless, I wanted to start a discussion here first as per the contribution guidelines.