more .hidden -> .visually-hidden

Bug 833534 (8ab27399) introduced `.visually-hidden`, made `.hidden` an alias in `sandstone/sandstone-resp.less`. For less files which imports only `sandstone/lib.less`, `.hidden` is broken. http://www.mozilla.org/en-US/about/partnerships/ is an example.

Or, maybe `.hidden` should be kept, as some times it make sense to hide some elements from screen reader, too?