use of @screen-xs-max in navbars.less

Created by: bassjobsen

@screen-xs-max is used to style the dropdown menus when collapsed. In the default situation this works cause @screen-xs-max = @grid-float-breakpoint-1 = 767. This breaks when changing @grid-float-breakpoint (b.e. to 0 for a non-responsive version).

Solution define a new variable: max(0,grid-float-breakpoint-1) and use this stead of screen-xs-max in navbars.less.

I don't know if the max() function works, see: https://github.com/less/less.js/pull/1371