Implement basic Group handling (#3)

Co-authored-by: Tim Lorsbach <tim@lorsba.ch>
Reviewed-on: enviPath/enviPy#3

epdb/models.py

@@ -43,6 +43,7 @@ class User(AbstractUser):
                                       on_delete=models.SET_NULL, related_name='default_group')
     default_setting = models.ForeignKey('epdb.Setting', on_delete=models.SET_NULL,
                                         verbose_name='The users default settings', null=True, blank=False)
+    # TODO remove
     groups = models.ManyToManyField("Group", verbose_name='groups')
 
     USERNAME_FIELD = "email"