[#68698726] Omg the backdrop users are coming

- Adds a backdrop user model (distinct from django users, just an email and manyToMany field for data_sets)
- Adds an import script for getting in new backdrop users from a json dump file
- Adds very basic admin interface with email based search (though we only have ~20 users to administer!)
  - this interface could be improved by adding better filtering in the admin interface for manytomany fields, but this seemed pretty tricky and out of scope for this, people will have to deal with it when adding users to data-sets for now
- Adds an API view for backdrop to chat up