Make the "Requested Changes to Prior Diff" reviewer icon red, not bluegrey

Summary: See PHI31. The "Accepted Older Revision" icon is (more reasonably) bluegrey, but that rule spilled over here where it doesn't make much sense. "Requested Changes to Prior Diff" remains in effect across updates, but the coloration implies otherwise.

Test Plan:
"Requested Changes to This Diff" (unchanged):

{F5092019}

"Requested Changes to Prior Diff" (now red, previously bluegrey):

{F5092020}

Note that the icons are different so this is technically colorblind-safe, and it's normally not important to distinguish between these two reds anyway.

Reviewers: chad, lvital

Reviewed By: lvital

Subscribers: lvital

Differential Revision: https://secure.phabricator.com/D18385