Make thumb_handler.php use MediaHandler::parseParamString

Use MediaHandler::parseParamString to parse the parameter string
of a thumbnail. This seems to make much more sense given the
design of the Handler class. Also mark ExtractThumbParameters hook
as deprecated.

This should be reviewed by someone familar with the 404 thumbnailing
infrastructure. I'm unclear if there was a reason for why it wasn't
using MediaHandler::parseParamString in the first place.

Bug: 49629
Change-Id: I43a0e7d6f0e45536e4e24b72140157791f5f027f
Merge "Make thumb_handler.php use MediaHandler::parseParamString"