Unique email addresses

This adds unique constraints but disables them by adding a column that is always NULL. Changing a config setting and running an admin command enables the constraints.

See #145 (closed) (issue should remain open until we make this mandatory in uffd v3)

TODOs:

• Migrations
• Decide on key function for email address comparison
  • Other software often treats email addresses as case-insensitive
  • In LDAP RFC4518 stringprep (case-folding + unicode normalization + whitespace normalization + ...) would be applied
• Decide whether or not to replace config key + command with just a command thats sets a feature flag in the database
• Tests
  • Models
  • CLI command
  • Views
• Translations
• Catch constraint violation when a new user is created in admin interface
• Refactoring:
  • Rename UserEmail.address_key to address_normalized and UserEmail.build_address_key classmethod to normalize_address, I think that is more descriptive
  • Clean up naming of cli command, feature flag, UserEmail.enforce_unique_constraint column