Rename OccupationListener to TopicPageCreationListener

This used to be used to create page & revision etc records
for new boards. Nowadays, however, that functionality is done
elsewhere, before we store the rest of the content (so we can
store the workflow with the associated page_id).

Nowadays, this listener is only used to create the Topic:Xyz
page for posts. We still need to do that to make sure that
core functionality like Title::exists works for Topic:Xyz
pages.

I've also removed the DeletedContributions hook's isEnabled
stuff. Now that we no longer "occupy" on load and occupy what
we used to, this no longer makes sense.

Bug: T105574
Change-Id: I460b3a835eaa74df76340d36fe350d272b92b4e3