Add builder_id to builds

[jameswestman]

We can set a builder_id on a token, and that ID will be stored on every build created with that token. We'll use this in flat-manager-hooks to insert an appstream tag for apps built by buildbot.

[jameswestman]

Hmm, thinking about this a bit more... This plan assumes that you can't edit buildbot builds with a direct upload token, which isn't a sound assumption if we allow switching from buildbot to direct upload. This needs more design. Maybe if a build is created with a builder_id, it can only be edited by that builder_id?

[jameswestman]

Or we could track builder_id on the build ref instead of the whole build.