commonElements: Import styling for <code> from mediawiki.org

Same theme as the styling for <pre> we already have (which also has
been imported from mediawiki.org, in Iea359250).

Source:
https://www.mediawiki.org/w/index.php?title=MediaWiki:Gadget-site.css&oldid=1057697

Redesigned <code>.
Should improve readability by not having the samples blend into the white background,
but by having a visible (though subtle) border around the code sample.

Originally authored by Krinkle:
https://www.mediawiki.org/w/index.php?title=MediaWiki:Gadget-site.css&diff=655396&oldid=648042

Change-Id: I48185e17a37e5c11b285e9a44176b47d59644bcd