Username not unique

[pascwhale]

Description

Multiple users are currently able to register an account using the same username, as long as they provide a different email address.

Expected Behavior

Multiple users should not be able to register an account using the same username, as long as they provide a different email address. Usernames should ideally be unique.

Actual Behavior

Multiple users are currently able to register an account using the same username, as long as they provide a different email address.

Steps to Reproduce

1. Register an account using a username and email address.
2. Register an account using the previous username and a new email address.
3. Both accounts will be allowed.

Possible Fix

Make sure username does not exist in database.

Additional Notes

This isn't mandatory, but it might be an issue depending on how trades are initiated in the future.