New features and some fixes

[FEATURE] Add new display mode "Time restriction" #482 (thanks @christophlehmann)

[FEATURE] Added event plausibility check #841

When an event has an enddate which is before the startdate, an error message will now be shown when the event is edited.

[TASK] Show visual warning for notification misconfiguration #774

[FEATURE] Referenced fields in CSV export #855
[BUGFIX] Ensure cache is flushed when expired registrations are handled #848
[BUGFIX] Handle non-existent event in checkCancelRegistration() #865 (thanks @christophlehmann)

[TASK] Make start- and enddate fields required in TCA #849
[TASK] Handle only own flexforms in DataHandler hook #858 (thanks @christophlehmann)
[TASK] Harden ArrayUtility and added unit test
[TASK] Removed all author doc comments
[TASK] Move extension icon (thanks @christophlehmann)