position 'top' for ext.flow.discussion

This seems to help the flash of unstyled content when you view a Flow
board.  (If necessary, `touch modules/discussion/init.js` to encourage
ResourceLoader to reload its cache.)

I had to change ext.flow.base.js to initialize mw.flow upon load instead
of later on document ready, because dependent parser and editor modules
access mw.flow upon load. It doesn't depend on or interact with the DOM,
so it doesn't need to be on document ready and clients may not expect
that API functions aren't available when this RL module is 'ready'.

The remaining FOUC I see is the flow header turning bold and dark gray.

TODO: think module loading through carefully, refactor so that only CSS
and JS that affects appearance of inital page from server is loaded
'top', maybe use addModuleStyles() for initial CSS applied to static
page HTML.

Change-Id: I79d57a9d6e77fba651a528116ef50ceb5ae82706
Merge "position 'top' for ext.flow.discussion"