Refactor #936

Updated by Alexander Watzinger 8 months ago

At the moment if an entity is update all links are deleted and linked again from form values.

For performance and data integrity integrety reasons only changed links should be processed.

We begin with the refactoring of dates and also simply the mechanism when we are at it.

* remove "exact date" system type and just use "date from value" without "date to value"
* refactor how entities getting dates (all at creation, remove get_dates function)
* refactor updating entities (dont delete/save if not changed)