The previous architecture did not work properly with the automatically
detected night theme, resulting in a weird mix of the night and day
themes on code blocks.
I'm not thrilled with the requirement this imposes that all of our
night theme CSS needs to be in one file, but we do need to get a quick
fix out here.
Fixes#15554.
This adds a parent selector, `rendered_content`, to night mode syntax
lighlight selector. This helps us in getting the "day mode" syntax
highlight styles in night mode.
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`.