Updated proofreading check marks to use sprites

Check marks for proofread have been modified to:
* Use a single graphical asset so that once loaded, status changes such as hovering do not introduce an additional delay.
* Emphasize the markers (bigger and more intense colors) so that they are easily recognizable.

Change-Id: I0d8593c59ef6f8e65a94c47839d6ba91b7089bc7
Merge "Updated proofreading check marks to use sprites"