Race condition in submissions and username changes can lead to incorrect message routing

[brassy-endomorph]

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

1. User A has username user_a
2. Anon user goes to /to/user_a and starts composing a message
3. User A changes their username to a_user
4. User B changes their username to user_a
5. Anon user submits a POST to /to/user_a
6. User B receives the message instead of User A

Describe the solution you'd like

1. For submissions include the user_id which is matched to the username in the URL
2. Better: implement Add a permanent record of userames to prevent squatting #648