Boilerplate code to allow adding editors

* Created ext.flow.parsoid & matching API module to support conversion in JS
* ParsoidUtils uses existing VE Api instead of calling Parsoid itself
* PHP always recieves wikitext (setContent), output arbitraty (getContent)
* Content can be saved as either wikitext or HTML in DB
* Multiple "editors" are possible; implemented right now:
	* none: wikitext-based; just a textarea
	* wikieditor: wikitext-based enhanced editor (though disabled)
	* visualeditor: HTML-based; rough VE standalone version
* Textarea's that should have an editor can call mw.flow.editor.load (and co)
* Already implemented editor for topic start, reply & summary
* Topic start PostRevision entry is now indexed
* ApiQueryRevisionContentFlow introduced, for querying latest revision content

The idea behind $wgFlowContentFormat (which defines how content in saved in DB)
is to make it possible for production wiki to save HTML content (to make output
cheaper - no wikitext->html conversion for every request), while still also
supporting wikis that are unable to run Parsoid (so everything basically works
on wikitext, and the only conversion that needs to happen is wikitext->html on
output and never html->wikitext for editing)

Sample 3rd party config:
* $wgFlowContentFormat = 'wikitext';
* VE is unusable; non-VE (wikitext editor) works

Sample config with VE/Parsoid enabled:
* $wgFlowContentFormat = 'html';
* VE & non-VE (wikitext editor) work

Change-Id: Ic6574b1602d515340f3fbdd341f1109b71bdaf99
Merge "Boilerplate code to allow adding editors"