Bug 882994 - [email] UI: Replace sync progress bar (including candy bar) / messages spinner with single spinning refresh button. r=jrburke

This is both a performance win (animations chew CPU, especially the candybar)
and a UX win since it gets rid of the confusing determinate progress bar
status.
Merge pull request #10407 from asutherland/email-progress-spinner

Bug 882994 - [email] UI: Replace sync progress bar (including candy bar) / messages spinner with single spinning refresh button. r=jrburke