Display (information about) the job blocking a refactoring job

When a page is renamed / moved or deleted, there is a lock system that locks the page tree that is being impacted by the rename action.

If another refactoring action (rename) is launched on the locked tree, a message will be displayed on the screen of this second action that the rename cannot start until that other job is finished.

We should also display (to restricted users?) some information about the job that is blocking the current job, which can be very useful especially if the blocking job is waiting on a question.