Require core edit permission to perform write actions

To detect if the core 'edit' permission was required we were checking
the button type and restricting all actions with 'button-type' !== 'GET'.
This turns out to be invalid because some actions, like edit-title,
draw 'GET' buttons because additional user interaction is required. In
this case the edit-title page will have its own POST submit, but that
configuration isn't a part of the $wgFlowActions.

The new method to detect need of the core 'edit' permission is to be 
explicit.  Each entry in the $wgFlowActions array must now contain the
key 'performs-write' set to a boolean.  Any action who's result is to
write to the wiki must set this to true so the edit permission is
respected.

No change is made to the 'button-type' key of $wgFlowActions, it still
refers to the type of button output in post.html.php

Change-Id: I1c4445305483e7b3bdf8ab455d23abc3b924b43d