settings: Make email required and username not
settings: Make email required and username not

Set couple settings to fine tune how emails and usernames are handled:

 * Require email address for logging in (ACCOUNT_EMAIL_REQUIRED) and
   don't require username (ACCOUNT_USERNAME_REQUIRED).

 * Make django-allauth to not use username at all in it's logic or even
   set it on social logins (ACCOUNT_USER_MODEL_USERNAME_FIELD), because
   username is set to not-so-user-friendly-UUID-based value in our
   `SocialAccountAdapter` anyway.

 * When authenticating via form, do that by email (and password) rather
   than username (ACCOUNT_AUTHENTICATION_METHOD).

 * Make sure that social logins query the email from the social account
   provider (SOCIALACCOUNT_QUERY_EMAIL).

mikkokeskinen/tunnistamo,mikkokeskinen/tunnistamo