Add ability to disable for anonymous users

This adds a new config variable, $wgVisualEditorDisableForAnons,
defaulting to 'false', which disables VisualEditor for not-logged-in
users. This only makes sense where the visualeditor-enable setting is
set to 'on' for all users; it will be used but briefly.

Bug: 50000
Change-Id: I7418c7d96a79b17e717abfba1af8c5df3141e591
Merge "Add ability to disable for anonymous users"