Allow regular expression control of plan branch deletion.

In BAM-18351, a flag to toggle branch cleanup for a particular branch was suggested as a new /plan/.../branch endpoint.  This would allow admins to programmatically toggle the cleanup status of plan branches.  However, it would also be beneficial to be able to fine-tune the plan branch deletion policy at creation time rather than have to go back after creation and push an API call.

In our use case, we have procedures that roll through a set of repositories and generate release branches.  This works well; however, because we cannot fine-tune branch expiration based on a regular expression, we need to go into each of n release branches and manually uncheck the cleanup checkbox.  We not only have to do this for releases but for certain long-running (but potentially idle) branches, which should remain in place until merged.

The ideal UI change would would allow for a regular expression to control expiration.  The current UI looks like:

• Delete plan branch
  •  After branch was deleted from repository
    • # days
  •  After branch inactivity in repository
    • # days

A more granular solution would look more like:

• Delete plan branch
  •  Only delete plan branches matching the expression:
    • (regex box)
  •  After branch was deleted from repository
    • # days
  •  After branch inactivity in repository
    • # days