releases: Enough with assumptions about tags

All the tags for a given release are now stored in the database.

This removes the need for all those properties, and all those
assumptions, which makes life easier for other deployments.
builds: Really untag builds
releases: Migrate the database to the new tag stuff

Caveat: This is not going to work with SQLite.
updates: Remove Update.get_build_tag

This method was flawed, in that it returned only one tag, when the
builds in that update can have many tags.

The only consumer of that method is now not using it any more, which
actually makes it simpler.
Merge pull request #59 from bochecha/tagstuff

Tag stuff