Add content type: fixed-html
API is drawing a lot on view-* APIs, both for displaying content
& editing content (we're now directly using the html output in VE)
We need to be able to request 2 kinds of HTML: the one including
our ContentFixer adjustments (for output) & the one for VE, where
we need the original content.
This patch adds a 'fixed-html' content type. 'html' will from now
be the original, unfixed, html - the way VE needs it & we store it.
'fixed-html' is what we'll output, where ContentFixers are applied.