Vendorize CSS sensibly

Rather than having some CSS lingering in directories like the static
base (icomoon) or vendored scripts (toastr, katex), create a dedicated
place for these under h/static/styles/vendor.

Icomoon and Katex fit together nicely, combining the font folders.

This commit also sees the re-introduction of the cssrewrite filter
to ensure that the relative URLs are properly rebased.

h/static/styles/.gitignore

-*.css
-!visualsearch.css
+/*.css