Add explanation in popover on disabled add-subscriptions input elements,
admin can't add subscribers to non subscribed private streams, only
subscribed users can.
Fixes#10593
When a user clicks the compose `+` button, create a popover at the
bottom right of the screen including buttons for opening a new stream
message or a new private message.
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`.
Rishi Gupta
Yashashvi Dave
Cynthia Lin
Marco Burstein
Akash Nimare
Aditya Bansal
Max Nussenbaum
Armaan Ahluwalia