Make Flow use mw-ui-constructive etc rather than flow-ui-

The flow-ui-constructive buttons are not loaded in new version of Flow,
only on cached pages, so let's update the appropriate buttons.

Change-Id: I4e125deb22e2de4f83ab8a6b3328480c9ae0b650
Merge "Make Flow use mw-ui-constructive etc rather than flow-ui-"