Gracefully handle situations where two or more users share the same email

[DavisRayM]

Bug

Currently during the login process if two or more accounts have the same email the authentication process fails with a 500 due to an uncaught exception.

Traceback (most recent call last):                                                                                                                                                                                                            
  File "/home/rapidpro/.virtualenvs/rapidpro/lib/python3.6/site-packages/rest_framework/views.py", line 492, in dispatch                                                                                                                      
    response = handler(request, *args, **kwargs)                                                                                                                                                                                              
  File "/home/rapidpro/.virtualenvs/rapidpro/src/ona-oidc/oidc/viewsets.py", line 175, in callback                                                                                                                                            
    user = self.user_model.objects.get(email=email)                                                                                                                                                                                           
  File "/home/rapidpro/.virtualenvs/rapidpro/lib/python3.6/site-packages/django/db/models/manager.py", line 82, in manager_method                                                                                                             
    return getattr(self.get_queryset(), name)(*args, **kwargs)                                                                                                                                                                                
  File "/home/rapidpro/.virtualenvs/rapidpro/lib/python3.6/site-packages/django/db/models/query.py", line 412, in get                                                                                                                         
    (self.model._meta.object_name, num)                                                                                                                                                                                                       
django.contrib.auth.models.User.MultipleObjectsReturned: get() returned more than one User -- it returned 2!