Train users should be able to perform failed package PPA rebuilds

[Łukasz Zemczak]

Currently the trainguards get poked whenever a rebuild in the PPA is required after a package FTBFS. This should be an option for the Train users, without them requiring special permissions in the silo archives.

[Łukasz Zemczak (sil2100)]

This is almost ready for staging. Just need to find time to finish up, push the branch somewhere and redeploy for testing.

[Robert Bruce Park (robru)]

Here's how I'd like to tackle this:

Need to resurrect that feature where build job only builds packages with new commits (this got lost in the porting effort from lp:cupstream2distro to lp:bileto).

Once that's in place, at the very end of the build job, it could just iterate over all builds in PPA and retry everything that isn't a success. That way users could click 'Build' once to build their packages, and then if they want to build the same commits again it would just automatically retry those builds without uploading new source packages.