When rendering math, the custom KaTex fonts failed to load due to the
relative URL references in `build/styles/katex.min.css` not matching the
actual location of the fonts in `build/fonts`. The KaTeX CSS assumes a
URL structure like:

```
katex.min.css
fonts/
  KaTeX_AMS-Regular.woff2
```

Previously font URLs in katex.min.css were rewritten from
`url('fonts/foo.woff2')` to `url('../fonts/foo.woff2')` by a PostCSS
plugin when building the CSS bundles. However this got lost when
replacing the client's local style-bundling script with the one from the
@hypothesis/frontend-shared package.

By relocating the fonts to a subdirectory of `build/styles` we can make
the asset references work without needing to do any rewriting of URLs in
the KaTeX CSS bundle.

In the process, the list of fonts included in the bundle was simplified
to include just WOFF2 fonts, since all modern browsers support that
format. Additionally an obsolete reference to fonts in src/styles/vendor
was removed.