SECURITY: Enhance CSS filtering in SVG files

* Filter <style> elements
* Normalize style elements and attributes before filtering
* Add checks for attributes that contain css
* Add unit tests for html5sec and reported bugs

Bug:69008
Change-Id: I732eece710f1bfaaeea1e5de541fcd4cfb375de7
Updated release notes and version number for MediaWiki 1.19.19

This is MediaWiki 1.19.19 security release.

Change-Id: I4d9513583ca233562e15b8e8a0518e6322a7f7a9
Merge branch 'REL1_19' of https://gerrit.wikimedia.org/r/p/mediawiki/core into REL1_19