Add mw-ui-* context colors for any element, plus special handling for anchors

Progressive, constructive, destructive can now be used independently of mw-ui-button.

Change-Id: I89d80927d603407678182592a1b12e2132de4141
(cherry picked from commit 5c74148ba3f7c4f64cca6902e516ebb0c6b7f52a)