Allow moving category pages

Allow moving category pages

Allow category pages to be moved. This is to preserve attribution of the
page only and does not cause pages in the category to recategorize to the
new one. A warning explains this when such a move is attempted. The new
right move-categorypages is required to do this, which is assigned to user
and sysop by default (the same as other move-related rights). The message
category-move-redirect-override can be used to cause custom text to be
placed on the "old" category page in lieu of a redirect.

Bug: 28569
Bug: 5451
Change-Id: Ic93616a54c8e98e3dc71daee3c92c466d64daffc