Allow to remove backport- label after successfully creating backport PR, allow to create backported- label

[felixfontein]

It would be helpful to have two (independent) options:

1. Remove backport-<branch_id> label after successfully backporting;
2. Add backported-<branch_id> label after successfully backporting (label's name should be configurable similar to the label for creating backports).

This allows to filter PRs to see

1. which were backported automatically,
2. where backporting failed and manual intervention is needed.

[webknjaz]

Ref pytest-dev/pytest#9555