Authentication: Fix integrity issues when editing a user profile with mixed case email addresses

[mauromsl]

Addresses some bugs:

• When a user edits their own email address, the user model is cleaned. If the username/email are in use the form won't validate.
• usernames won't be set to mixed case values when editing an account anymore
• When an editor edits another user's email address, the user model is cleaned.

Adds authentication tests for:

• Avoiding registration of accounts with the same email address with variations in the casing
• Ensuring the email address field in the login system is case insensitive.