Collection branching

[msimacek]

Branched collections are created from a master collection (rawhide). It
performs these steps:

1. creates a branched collection with the same attributes as the master
   one, except for display name and bugzilla version passed as arguments
2. copies most recent (1 month) builds from master collection to the
   branched one
3. sets new name for the master collection and moves it to give new koji
   target

Example invocation:
koschei-admin branch-collection f27 f28 -d 'Fedora 27' -t f28 --bugzilla-version 27

[mizdebsk]

Looks good. I deployed and tested this in staging:

# koschei-admin branch-collection f26 f27 -d 'Fedora Rawhide' -t f27 --bugzilla-version 26
# koschei-admin edit-collection f26 -d 'Fedora 26'

It worked, but display name edit was necessary.