Embargo: prevent activity stream items and logging from non-embargo users

The EmbargoWorkflowEventsProcessingModule triggers the embargo worklfow even for users who are not embargo users. This causes logging that embargo has been set on a document, which is also reported on the dashboard.

The EmbargoWorkflow does a check if a user is allowed to set embargo. This should (also) be done in the EmbargoWorkflowEventsProcessingModule.