Steps to Reproduce

1. Create a user with some vowels in the username
2. Create another user with the same username, but change some/all of the characters to their accented equivalents (e.g.: online-user and önlínê-user)

Expected Behavior

The users exist in both mitxonline and Open edX

Actual Behavior

For unexplained reasons, edX treats both of those usernames as the same, so only the more recent user will have a User record on the Open edX side, and one or both users will experience some of the auth/enrollment issues that we saw before we patched up username creation and validation

Related Issues

#202
#244

Implementation

1. Add a new field to the mixonline User model (normalized_username or something like that)
   • Should be indexed and unique.
   • Should have a data migration that fills in this field for all existing usernames. The value should be the current username with accent characters 'normalized' to unaccented versions.
2. When validating a new user, the username should be 'normalized' to an unaccented version and checked against this new field. If the same normalized username already exists, the user should see the same validation message that they would see if another user existed with the exact same username value
   • Example: user with username üser1 already exists, then someone else tries to create a user with username user1. That should trigger a validation error, and they should see the message Username 'user1' already exists or whatever the validation message is currently for an exact-match username collision. It's not necessary to tell them about the accented character constraints – we should be fine with just telling them to try a different username