Make mandatory metadata required

[IkramMaalej]

Part of #4322
Fixes #4377
Fixes #3403

[andre-hohmann]

@IkramMaalej : It seems as there is still no check for lists with checkboxes, of which at least one must be selected. In my tests, it is the list of "Digital Collections". See example below.
I can create a process without selecting a digital collection, but when i validate the process in the metadata editor, i get a warning, that no digital collection is selected.

I cannot assess the behaviour with drop-down-fields, because in these mandatory fields (script, rights information, ...), one value has to be chosen. It is not possible to select an "empty" value. This might be described in:

• Process creation - no check, if values are available for mandatory meta data (for "option" meta data) #4377

Other mandatory text-fields, as for example TSL/ATS, CatalogIDDigital, ... are validated correctly.

Example

[andre-hohmann]

@IkramMaalej:
The validation works now. Processes cannot be created, when mandatory checkbox lists are not selected.

However, there are error messages, which cannot be interpreted - see example below. Either, the text of the messages must be adjusted, or the messages must be hidden.

In my opinion, the error messages can be hidden, because of the two following arguments:

1. There are no error messages, when mandatory text fields are empty.
2. The affected fields are highlighted by red lines and can be identified.