Do not allow a user to delete a page they can't edit

This was probably overlooked in the past because usually the only users
who can delete pages also have permission to edit the relevant
protection levels.

Change-Id: Ibe28a69c9fbab00b81c53b1643df722a3f1fbf19
(cherry picked from commit 002a27790137e91b124ffeed6efc8dbe833e1dab)