Expand "Become an Alpha tester" with a focus on the Tor forum instead

I've rewritten and expanded the Become an Alpha tester page within community/user-research with an aim to redirect new Alpha testers to the Tor forum instead of the legacy tor-qa mailing list. These changes are required to support the Community QA recruitment campaign outlined in tpo/applications/team#6, and will close #269 (closed) specifically.