On category pages, only replace category description when saving

Instead of overwriting everything else on the page (e.g. 'Pages in category ...')

See also bug 65349 / Ib6c49286 for file pages with a similar issue

Bug: 64239
Change-Id: I59ff9de5d0463f0f1ae8a18d54ebea5844fb1af5
Merge "On category pages, only replace category description when saving"