Occupy talk pages!

1% of the talk pages have 99% of the discussion!

You can "occupy" (terminology to be adjusted) talk pages either not
at all ($wgFlowOccupyPages = false), for all pages (" = true), or
only a given list (" = array( 'X', 'Y', 'Z' ))

Includes a refactor of display code.

Known issues:
* Currently we refuse to show workflows for pages that don't exist.
  I've worked around this by not taking over pages that don't exist.
* Code to initialise view layer is currently somewhat duplicated.
* We use RequestContext::getMain(), whereas instead we should really
  accept a RequestContext from the MediaWiki object.

Change-Id: I3d0f5eda696a1385c0b986311fdb4480e2f7921a
Merge "Occupy talk pages!"