Migration Plugin

The data in the repos collection needs to be migrated into the repository table in postgres.

For each field in the repository document:

repos               =>  repository
____________________|____________________________
                    => id (generated)
repo_id             => name
display_name        => (omitted: redundant with name or description and don't think anyone is using this)
description         => description
notes               => notes (by Notes relation)
scratchpad          => scratchpad (by Scratchpad relation)
content_unit_counts => (omitted: calculated by content_summary())
last_unit_added     => last_content_added
last_unit_removed   => last_content_removed
_ns                 => (omitted)