Instead of modifying a page rendering hash,
use the proper mechanism of parser option
registration. The cache is still split on parser
option value. The 'math' option is lazy, since
not all pages contain 'math' tag and we want to
avoid an user preference lookup for every parse.
WARNING: this will invalidate ParserCache for
all pages that contain <math> tags, but according
to Moritz, this is about 1% of all pages, so we
should be fine.
Setting up the test is still rather hard and weird
due to very hacky rendering mode to user option
converter, and math renderer interaction with the DB.
This will be cleaned up later.
Depends-On: I92b3e18fabef4eecac2ec2a4844f1be2716e5d89
Change-Id: I3bce04684070ad306685dabbc51267def25773cd
