mediawiki.content.json.css: Don't use funny fonts

* Allow skin-provided fonts for .mw-json.
* Use standard monospace font for .mw-json .value. We need to write
  'monospace, monospace' because of old browsers, see design/monospace.html.

Change-Id: I4ed850408895dd1962a0503227ccb384bf479790
Merge "mediawiki.content.json.css: Don't use funny fonts"