Enable admins to easily add new experimenters to allowlist

Right now, the only way to add experimenters to the allowlist is by manually adding their email address (after manually ensuring it's all lowercase) as a new document under our database's allowlist collection in the Firebase console.

Ideally, we should:

• Add a textarea/button to the admin dashboard that allows admins to submit new experimenter emails (automatically lowercased) to the allowlist
• [optional] Enable admins to remove experimenters from the allowlist via the admin dashboard

Related: We could consider enabling experimenters to request access via the web app (e.g., on the home page when not logged in) such that the requests appear in the admin dashboard for admins to easily review and approve/reject. (Our current solution is directing requests to a Google Form, which we manually check.) However, not sure admins are more likely to check the Deliberate Lab admin dashboard over form responses.

Start by locating the admin dashboard and the Firebase allowlist collection, then trace how admin actions are authenticated and persisted. Implement the required flow for submitting experimenter emails with automatic lowercasing; consider removal and request-review features separately because they are optional or exploratory. Done means an admin can add an email through the dashboard and it appears in the allowlist.