move the navbar-collapse transition to xs

avoids stacked navbar on narrow (but not too narrow) windows
collapse the navbar on narrow windows

so it doesn't take up a bunch of vertical height when not in use
tweak collapsed navbar style

- notification and indicators appear above menu on small screen
- collapsed menu button floats left
move grid-float-breakpoint to 540px

collapses navbar before running over to two lines
due to help/indicator collision.
Merge pull request #6538 from minrk/navbar-collapse-xs

collapse navbar on small screen