Don't repeat label styling

Only have one selector for .mw-ui-vform > div label , move all CSS to
that.

Backported from master for new opt-in Create account form.

Bug: 47532
Change-Id: Ibd36f818865aa5ec99cc187bef983261485966d3
(cherry picked from commit dd428d9fdf5231df2a1b1481dbec7d37cd0c063c)
Merge "Don't repeat label styling" into wmf/1.22wmf2