Non-hacky way to feed title to Parsoid

Gets rid of $wgFlowParsoidTitle for the purpose of calls to Parsoid.
It still exists for Redlinker - needs to be removed in there some day too.

This introduces some methods in Collection to fetch associated Workflow & Title
objects. When calling this on multiple collections, these should preferably
already be batch-loaded in advance (both treeRepo & cache::getMulti will local-
cache those results efficiently). I'll submit a bug to build some batchloader
classes - that's too tedious now.

Bug: 58664
Change-Id: Ifc5c6f3f79890ec59d438b0eec004941c318b214
Merge "Non-hacky way to feed title to Parsoid"