Usernames should be optional for new users

[jace]

When we receive a new user, the profile form is a roadblock preventing them from moving on until they pick a username. However, (a) unique usernames get harder to pick over time and (b) there's no practical utility to having a username at this time as there are no profile pages.

Usernames should remain optional in the new user profile form and should perhaps not be present at all.

Alternatively, usernames should be auto-generated and guaranteed available (for LoginProvider logins) so that the user can continue without the cognitive load of selecting one.

[jace]

Usernames became mandatory in 413d19b in Nov 2012.

[jace]

With #232 username management will move entirely into a client app. For now, we should go back to making usernames optional.