Add HiDPI PNG variants for 'Powered by MediaWiki' footer icon

Add HiDPI PNG variants for 'Powered by MediaWiki' footer icon

Added 1.5x and 2x-density variants of the 'Powered by MediaWiki' footer
icon, built from this SVG version on Commons:

  https://commons.wikimedia.org/wiki/File:Powered_by_MediaWiki.svg

Note the SVG version is too large to use directly right now as it
contains a huge amount of detail; the PNGs are only a few KiB.

Renderings from Commons taken and run through pngcrush.

Note that adding 'srcset' attributes to footer icons appears to work
just fine here, so can be done for others.

Bug: T65872
Change-Id: I785d21add456eeddb6ed1ee36a1906d178323e63