Migration Plugin

The data in the repo_content_units collection needs to be migrated into the repository_content (join) table in postgres.

For each field in the repo_content_units document:

repo_content_units  =>  repository_content
____________________|____________________________
                    => id (generated)
created             => created
repo_id             => repository_id (by Foreign Key relation)
unit_id             => content_id (by Foreign Key relation)
unit_type_id        => (omitted: see [1])

[1] The master-detail modeling of content means that every content (unit) has a record in the master table and
thus a unique id. The combination of id and type is not longer needed.