Support updating search index when page is deleted

Right now, if you delete a page then you end up with a stale
entry in the search index, this affects all core SQL-based
searches.

For extensions, this means they no longer have to implement
something like ArticleDelete, they can just add the delete()
method to their SearchEngine subclass.

Change-Id: I9d8a9878aeebc53f453ab1cbacc03fe73fcca949
Merge "Support updating search index when page is deleted"