PR triggers should be more flexible about their target branches

[Matt Mitchell]

PR triggers today can apply to a specific branch. We don't easily allow through the Utilities to apply to multiple branches. This should be fixed.

In addition, a change to the ghprb should be made that can allow for the trigger to NOT apply to specific branches.

The other tracked branches for a repo should also be passed into the generators, so that logic like:

'Apply this PR trigger to master and to any other branch that isn't tracked'

[Doug Ilijev]

👍

[Matt Mitchell]

This is fixed by recent contributions to the GHPRB, pending merges and upgrades on our end.