* adds initialize_objects to an abstractmethod of the super class to be implemented in subclasses
* differentiates error messages if intervention does not exist or is just not shared with the user
* adds support for POST of new compensations
* adds shared_users property to BaseObject and Compensation to simplify fetching of shared users (Compensation inherits from intervention)
* extends compensation admin index
* modifies compensation manager which led to invisibility of deleted entries in the admin backend
* fixes bug in sanitize_db.py where CREATED useractions would be removed if they are not found on any log but still are used on the .created attribute of the objects
