Implement backporting actions
There are various backport DAG shapes we use. A tool should be available to create these in some way (or at least bring together the various bits we need to make the end result).
It should:
- be able to make the relevant branches
- support creating the relevant
Backport: foo
trailers necessary for the branch - take a branch and make it available for a release branch (rebasing it back; see #73)
- in the case of conflicts, it should be able to use the first item in the list to bring the two together as necessary
Cc: @brad.king @tjcorona