Add webhook for GitHub pull request merges

[nanaya]

No frontend update yet. All merges will appear in main changelog page. For builds, it'll only show title with link to pull request.

Admin frontend also not happening yet because I ended up complicating it ┐(' ヮ' )┌

Setup:

• env file:
  • CHANGELOG_GITHUB_TOKEN, generate with any password generator or whatever
• github webhook:
  • webhook url: https://<domain>/home/changelog/github
  • event type: pull requests (or all)
  • secret: value of CHANGELOG_GITHUB_TOKEN

Showing in build, table build_changelog_entry:

• build_id: build_id of the build in osu_builds
• changelog_entry_id: id of the changelog entry in changelog_entries

Stream for web (new): call it web in streams table in osu_updates database for purple color outline. Don't forget to update UPDATE_STREAMS accordingly.

In github_users, set user_id to link user to osu! profile instead of github. User can be preemptively added by setting either user_id (osu! user id) or username (github username) or both.