This refactors the .message_controls to stop relying on absolute
positioning and strange CSS, and throws them inline.
This also restyles so they hang to the right of the time which is now
always present.
Fixes: #3761.
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.