This change prevents situations where specificity causes some rules to
be overridden by the ones intended for mobile.
* Added mixin `responsive-invisibility()`
* Swapped out `display: none !important;` for new mixin

* Any mentions of .btn-mini
* Old docs page that somehow found it's way back

`img`s are no longer responsive by default. Instead, use
`.img-responsive` as a class or mixin. Existing `.thumbnail > img` and
`.img-thumbnail` classes now make use of this as a mixin as well.

fixes #7961 a bit: adds css for extra tooltip placement options. /cc @fat you just need dat js prolly

remove submenus. for more context: http://www.youtube.com/watch?v=ggWyUEuGcWY&feature=youtu.be&t=48s

Prevents an odd alignment issue in mobile views by forcing the nav
links to be full width without undoing the float.