Trimming now happens in the general validation function.
Added deeply integrating tests for the member check-URLs, sharpened the remaining tests
If there are no activities at all, activitystore.activitiesForGroupIdsAndRegisteredMemberId now intercepts the mongo error and returns an empty list.
Adapt announcement tests to new trimming workflow.
Merge pull request #720 from NicoleRauch/validation_tests

Validation tests