Add a permanent record of userames to prevent squatting

[brassy-endomorph]

Is your feature request related to a problem? Please describe.

Acme Co. might claim the username acme, put links to their inbox on their website, then change username or delete their account. Someone else might then attempt to claim the username in the expectation that the URL on Acme Co.'s website stays live and drives unsuspecting users to their inbox.

Describe the solution you'd like

At the very least verified usernames should go on a list so they can never be claimed again. Maybe for all user names.

Maybe we disallow changing usernames at all and make it very clear that usernames are permanent.