#229 Shared users mandatory on admin

* changes mandatory state of users and teams on admin backend to optional (as expected by the model)
* adds team selection to admin backend

konova/models/object.py

@@ -434,8 +434,16 @@ class CheckableObjectMixin(models.Model):
 
 class ShareableObjectMixin(models.Model):
     # Users having access on this object
-    users = models.ManyToManyField("user.User", help_text="Users having access (data shared with)")
-    teams = models.ManyToManyField("user.Team", help_text="Teams having access (data shared with)")
+    users = models.ManyToManyField(
+        "user.User",
+        help_text="Users having access (data shared with)",
+        blank=True
+    )
+    teams = models.ManyToManyField(
+        "user.Team",
+        help_text="Teams having access (data shared with)",
+        blank=True
+    )
     access_token = models.CharField(
         max_length=255,
         null=True,