AphrontBars

Summary: Aphront widgets that render the either a discrete or continuous value as a horizontal shape.  Like a progress bar, or a five-star rating bar.

Test Plan:
`/uiexample/view/PhabricatorAphrontBarExample/` ...which shows this, amongst other things:

{F33898}

Reviewers: epriestley, chad

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2094

Differential Revision: https://secure.phabricator.com/D5122