Change mw-ui anchors to require mw-ui-anchor base class, and introduce mw-ui-text base class

Introduced mw-ui-text base class for plain-text, non-interactable elements which require context colors (eg. icons)
Also added mw-ui-anchor for interactable elements (such as anchors)

Note: This is not supported by IE6 at current time.

Bug: 69212
Bug: 70801
Change-Id: I4d017d0a22cb4f3ca52b6228e45c0463c110ae64
Merge "Change mw-ui anchors to require mw-ui-anchor base class, and introduce mw-ui-text base class"