DeploymentCommand: add user-group-css task for user group CSS

Some messaging on the wikitext fallback pages needs to be shown to anonymous users, while other text needs to only be shown to logged-in users. This MR adds the npm run user-group-css command to install a User group CSS system.

This is related to T363221 but is not distinctly for that task; there are other messages (edit notices, etc.) that will need user group CSS.

Bug: T362809

Bug: T363221