File view: Replace drop-down form with buttons for view options

Summary:
It always bothered me that adding/removing blame view takes two clicks.
Showing it like this saves a click for almost all transformations, and I think it feels nicer too.

I'm open to adding a user-setting for this (This form or the drop-down), but figured I'd ask first.

Test Plan:
Use buttons to switch modes - make sure text on button matches action.
Repeate with lines highlighted.

Reviewers: epriestley, vrana

Reviewed By: epriestley

CC: jungejason, Two9A, aran, Korvin

Differential Revision: https://secure.phabricator.com/D3499