This better sets expectatations for the fact that in Zulip, the
Organization settings UI is available read-only to non-administrator
users.
Tweaked by tabbott to update some additional references.
Create `media.css` using media queries that had been at the bottom
of `zulip.css`, then update miscellaneous setttings/docs files.
I also add `.screen-medium-show` and `.screen-narrow-show` to
`media.css`, as they seem to be an important part of our
responsive design.
Fixes#1532.