Add a primitive de-duping feature when getting local pagelinks.

Since we automatically convert some namespaces through $wgNamespacesToConvert,
a user could add [[User:Admin]] and [[User talk:Admin]] to a spamlist, resulting
in duplicate notifications. Now it stores the title as they key of the array,
which will automatically clear out any duplicates.

It also automatically follows all redirects to avoid duplication that way.

Change-Id: Ib642961f1e74b3d3fe64e18e8717dc6d1f0dd18b