detect "Fixes" lines in commit messages, close mentioned issue

[dmitshur]

When someone merges a commit to the main branch of a repository on https://dmitri.shuralyov.com, its commit message should be scanned for any "Fixes" lines, and said issues should be closed. (See dmitri.shuralyov.com/website/gido/...@5bb3cef2 for a recent example where this would've been helpful.)

This requires:

• coming up with a well-defined syntax for the "Fixes" lines
  • it needs to be compatible with indieweb URLs
• implementing code to parse commit message and detecting "Fixes" lines
• performing an issue service API call to close said issue

Once implemented, similar functionality can be used to help resolve golang/go#29599.