Use CONTENT_FORMAT_JSON instead of "json"

The content format is used only internally,
but is exposed in the documentation of 
API's action=edit. On MediaWiki.org (and other
Flow wikis), a format of "json" is displayed,
in addition to "application/json", which is what
Flow should be using.

Some backwards compatibility code is necessary
to make sure existing pages don't break. This is
also what Scribunto does.

Change-Id: Iffe01771c775222f0e885f998f7e5b20a6092e11
Merge "Use CONTENT_FORMAT_JSON instead of "json""