CORE-5797 Added avus table user ID conversions.
CORE-5797 Refactored find-existing-metadata-template-avu.

Refactored an avu->where-clause function out of the donkey.persistence.metadata/find-existing-metadata-template-avu function.
CORE-5797 Updated template avu endpoints.

Updated the donkey.persistence.metadata functions for template AVUs to set and update the created_by and modified_by columns instead of the owner_id column.
Updated the Metadata Template AVU CRUD endpoints to no longer use the owner_id field, to no longer return the "user" field in JSON responses, and to check for write permissions for the create, update, and delete endpoints.