support creating "merge proposals" in the Debian BTS

[jelmer]

This would involve filing bugs with patches attached.

If the package has an associated Vcs-* header, we could use that to check when the patch was applied.

If it doesn't, then an upload (i.e. a Version being set that fixed the bug and the bug being closed) that includes the patch would be a good indicator.

If the bug was closed without any of these things being true, that would be the equivalent to "closed".

Finding relevant bugs back after the fact could be done with user tags (i.e. the equivalent of "branch names" in silver-platter terminology).