Tighten up the install button to spec

[muffinresearch]

Fixes: mozilla/addons#9690
Fixes: mozilla/addons#9689

• Buttons are now actually round.
• Button gaps are on spec
• The width of the background is to spec.
• The hover states are removed for uninstalling/installing and disabled states.
• The active button handle is in the right position as opposed to being too far to the right.
• Tick image allows mouse events to pass-through: Fixes The cursor changes when mousing over the "tick" following a successful install addons#9659

Before:

After:

[coveralls]

Coverage remained the same at 100.0% when pulling 37f970e on muffinresearch:fix-errant-install-button-styles into 9b27874 on mozilla:master.

[mstriemer]

These changes look good. It looks like the switch is animating over by changing the left value. Would a translateX($size) work there instead? That should make it render more smoothly. Also I've noticed that sometimes the animation janks a little and the border turns green and then the background does, is a transition missing on the border?

r+wc

[muffinresearch]

I'll land this one, and do a new issue to tune up the animation - good points. Thanks!