#31 API Tests

* writes test for sharing using the API
* fixes bug on frontend form where an exception occured on generating a new API token if no token existed, yet
* adds permission constraint (default group) for using the api in general
* fixes default-group-only behaviour for sharing-API, so users can only add new users and not removing them, as long as they do not have any other group membership like registration or conservation office
* changes 'ksptoken' to 'Ksptoken' to match CGI standard for http header keys

user/forms.py

@@ -152,7 +152,8 @@ class UserAPITokenForm(BaseForm):
         """
         user = self.instance
         new_token = self.cleaned_data["token"]
-        user.api_token.delete()
+        if user.api_token is not None:
+            user.api_token.delete()
         new_token = APIUserToken.objects.create(
             token=new_token
         )