Bug 1021981 - [email] Update toast display to 2.0 visual redesign

This converts the toast display to fit in better with the 2.0 redesign, a gray background (#5F5F5F) with white text.

* Removes the cancel button on the left, as it was just a line separator and just looked confusing. Now just tapping on the toast will dismiss it. I confirmed that tapping on one of the buttons still executes the button pathway.
* Uses flexbox to lay out the message with the button.
* Changed the .msg-list-topbar (N New Messages blue bar display) to use the same blue as the unread dot, to better match the 2.0 visual refresh. The old blue was slightly off.