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.
Some node tests used to pass as long as prior tests ran,
but then they would fail if you ran them standalone. Now
we are more aggressive about cleaning up node's require
cache after each individual test runs.
This introduces a very minor different in behavior if you specify
an invalid filename as a command line argument. We now show
warnings for those *before* running the rest of the tests.