Follow-up 23cfe1bba: harmonize image embedding CSS

Follow-up 23cfe1bba: harmonize image embedding CSS

23cfe1bba changed ResourceLoaderImageModule::getCssDeclarations(),
but contrary to the documentation did not synchronize it with
mediawiki.mixins.less. This commit does that.

This commit brings the -o-linear-gradient rule back (this time with
a comment that explains why it's there), and removes the
-webkit-linear-gradient rules from mediawiki.mixins.less
and CSS code that copypasted it.

Change-Id: Ia7e9738d90919248748e6968192bf02e420cddd0