Primary buttons (such as the Log In button) have been a sort of dark

blue/grey in Review Board for a while now. RBCommons replaced this with

a brighter blue button, which feels more fresh and clean, and is a

better match for the rest of our UI.
This change adopts that button color (basically -- slight change in the

gradient) for Review Board.

Tested the buttons in the UI. We've actually been testing this style for

years on RBCommons.
Made sure the hover effect still worked.