This is a fixup for e1291cf839.
While copying the the rules of `#searchbox` to `#searchbox_legacy`
in the search pills feature, the existing `#searchbox` rules were
missed in the conversion.
`#searchbox_legacy` has been added beside `#searchbox` in `media.scss`
instead of replacing that as both of them need those rules for the
mobile view.
This commit transitions all styles in app.css in the Django pipeline
to being compiled by webpack in an app-styles bundle, and renames the
various files to now be processed as SCSS.
To implement this transition, we move the old CSS file refernces in
settings.py and replace them with a bundle declared in
`webpack.assets.json` and includedn in the index.html template
Tweaked by tabbott to keep the list of files in `app.css` in
`webpack.assets.json`, and to preserve the ordering from the old
`settings.py`.