Placing the text overflow on the span instead of the anchor element
prevents issues like T287522#7272991 from occurring.
Additionally:
* Uses the `text-overflow` mixin on every toggle list item label instead
of just the user link item as it simplifies the logic and presumably
we'd want any text that overflows the menu to be handled the same.
* Changes the anchor element to use flexbox/align-items to vertically
center instead of relying on `vertical-align` both the span and the
icon. Eventually, this should be put into core (see
I029f97ba9d5e7f46c8aa175d9a6bbb45ef9615df) but we have to remove all the
overrides that use vertical-align first.
Bug: T287522
Change-Id: Ie0fbff9dfaf8444c76125df52931a687730b4ad1
