Capture allow-list requests in Checkmate's admin pages

Instead of just making users send us an email when they want a site to be added to the allow-list, collect these allow-list requests in Checkmate's DB and enable staff to see the list of requests and allow and deny them in Checkmate's admin pages.

Note that we would probably still want to send an email notification when a request comes in (with a link to the admin page where you can allow or deny the request), unless we just require support staff to regularly check the admin page.