Add option to retain completed transitions

Imo it makes sense to not include processed transitions in the count as that retains the current behaviour (i.e showing the count of transitions yet to be processed) but config can't hurt I suppose

As discussed offline with @dpi I don't think the config is necessary, let's just always only show the pending count. This is inline with Gmail, the tab title is "Inbox (1)" when you have 1 unread message, regardless of the number of messages in your inbox. Folks can always extend this and override ScheduledTransitionsLocalTask::getEntityTransitionCount or we can explore other options in the future if there is demand.

This needs updated test coverage.

Patch for 2.3.x

Tentatively marking this as RTBC with one small question on the MR. I think this is otherwise looking very good!

Minor tweaks, green tests.

As found in 2.4.0-beta1

Automatically closed - issue fixed for 2 weeks with no activity.