Add mw-ui-icon

Compatible with IE8+ (because of the use of pseudo-elements).
This includes support for icons on the left of the element and
icons which hide the text of the containing element.

I attempted to make an mw-ui-icon-element which didn't use pseudo
elements but the problem with this is how it plays with things such
as mw-ui-button and the gutter. I'd suggest exploring this later as this gets greater
adoption.

In addition to this I have dropped mw-ui-icon-after since I don't see any
clear use cases for this yet and I think it is overengineering the solution.

Bug: 55535
Change-Id: I68a1b207d8a4af57879361921a5f8c3aeda3fd9a
Merge "Add mw-ui-icon"