Move Tester role permissions to alternative allowlist instead of w3c team membership

[s3ththompson]

Per w3c/aria-at#436, we want to avoid having to add Testers to the w3c organization (which rules out using w3c teams to handle role membership).

As discussed in the May 20 community group meeting (minutes), we will keep Admin role management under w3c teams but devise an alternate allowlist for Testers.

To begin, this could be a list that is maintained as part of the w3c/aria-at repo (either a wiki page or a checked-in allowlist). In the long run, we should consider an in-app approval mechanism which would greatly improve UX and make it easier for Testers to request the right permissions.

[s3ththompson]

Closed by #304