Reset $wgTitle in AbuseFilter::filterAction()

If AbuseFilter::filterAction changed $wgTitle, we need to reset it back
to the old value before returning.

And for good measure, we also need to check the main RequestContext to
make sure it didn't cache the bogus $wgTitle on us.

Bug: 53498
Change-Id: I4d706a1ee07f0b171f98e7d558d7be27e6759614