Update all references to user when username is changed

• Currently if you change your username, none of the references are updated. Clicking on a reference to the old username goes to a 404 page.
• If somebody creates an account using your old username, do all previous references to that username link to the newly created account?
• We could limit the number of times users can change their username (for example, a 60 day lock until it can be updated again)