First, it's silly that these weren't in common.css in the first place,
since that meant these were a bunch of duplicated code, but
additionally, that meant that these weren't available on the
`/activity` page (or other pages that don't include the portico styles).
Fixes#9561.
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`.
Tim Abbott
Aman Ghebreweldi
Armaan Ahluwalia