fix bug 753563 - Support username changes

Users now find an extra field on their profile edit page titled `Username`. Changing this
field allows them to change their username.
Existing usernames that do not comply with the new validation rules will not be affected.
Once you've rebuild this to be a field in the `UserProfileEditForm` class simply fetch the username with `username = profile_form.cleaned_data['username']` and make the `username_changed` check. If true, modify the `profile.user.username` field.