This consolidates the photon styles into the old pre-photon styles, simplifying the CSS.
As a result, it fixes the layout of the bookmarks menu as well. There's still some work
left to do there to make submenus align properly with their parent once opened on
Windows, but that isn't a regression caused by this bug so I am not addressing it here.
I have deliberately not updated the checkmark besides aligning it properly on OS X - it
seems we need more design details as to how that should look in the sidebar header menu
as well as the bookmarks panel before changing that further (if indeed it does need
further changes).
MozReview-Commit-ID: JPl2VwUh3e1
c947c1b510