BAC-1285: Send unique URLs to the purger queue

* introduce ``ScribePurge`` class and remove most of our changes from core ``SquidUpdate`` class
* introduce ``RestInPeace`` hook triggered just before MediaWiki ends its job
* queue all URLs added via ``SquidUpdate`` class
* remove duplicates and send unique URLs on ``RestInPeace`` hook in ``ScribePurge`` class

@prein / @michalroszka