Many of them are now automatically generated by autoprefixer, while
others are unnecessary based on .browserslistrc, and some were just
wrong (the linear-gradient based checkerboard pattern in lightbox has
been broken in Firefox for a while).
Signed-off-by: Anders Kaseorg <anders@zulipchat.com>
This moves all the stylesheets like stats, billing etc. to another
directory called `static/styles/portico/`, matching the directory
structure of our JavaScript.