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.
Currently (https://github.com/rtfd/recommonmark/issues/3)
the recommonmark bridge (which allows Sphinx to read
Markdown) does not support tables, so the directory structure
doc is now a bulleted list instead of a set of tables.