Anonymizing/Scrubbing Metadata (createdDate, createdByUserId)

The current RMB implementation of metadata (createdDate, createdByUserId, updatedDate, updatedByUserId) resides in RestVerticle.java and the database triggers in metadata.ftl and has these capabilities:

Removing createdByUserId works (requires an UPDATE command for both created_by and jsonb->'metadata'->>'createdByUserId' though).
Removing updatedDate works.
Removing updatedByUserId works.

Removing only createdDate or removing both createdDate and createdByUserId doesn't work:
On each subsequent update updatedDate and updateByUserId are written to createdDate and createdByUserId.

In addition the metadata triggers are implemented in a complicated way using the two extra fields creation_date and created_by. These fields are not needed because the trigger automatically provides these values as OLD.createdDate and OLD.createdByUserId.

Suggested solution:

Rewrite the metadata triggers without the extra fields.
Make createdDate an optional field (currently: required by metadata.schema).
Allow to remove any of the 4 metadata fields.
createdDate and createdByUserId remain unset on subsequent updates after they have been deleted.
Explain how to anonymize/scrub the metadata fields in RMB's README.

Open question:
Currently anonymizing/scubbing metadata via the PUT endpoint does not work because RestVerticle overwrites the values.
Do we want to allow the PUT endpoint to remove some metadata field by allowing a null value in any of the four fields to indicate that the field value should be removed? If yes, should this require special privileges ("metadata.delete")?