(CE-1067) Apply MW 1.19.19 security release

This applies the MW 1.19.19 security release which applies extra
CSS filtering to SVG files to prevent the inclusion of third-party
content.
(CE-1067) Add MW 1.19.19 release notes and update version number

Add the release notes for the MW 1.19.19 security release and update
the version number.
Merge branch 'CE-1067' into dev