If we want a dashboard with a history of stagings, maybe not deleting
them would be a good idea.
A replacement for the headless stagings would probably be a good idea:
currently they're created when splitting a failed staging containing
more than one batch, but their only purpose is as splits of existing
batches to be deactivated/deleted to be re-staged (new batches &
stagings are created then as e.g. some of the batches may not be
merge-able anymore) and that's a bit weird.
AL thinks it's not useful and it's better to always squash/rebase a
single commit & merge multiple. Mark tests as xfail'd instead of
removing them.
Also mark test_edit_retarget_managed as skipped explicitly
* p0 cancel existing stagings in order to be staged as soon as
possible
* p0 PRs should be picked over split batches
* p0 bypass PR-level CI and review requirements
* p0 can be set on any of a batch's PR, matched PRs will be staged
alongside even if their priority is the default
