POC: Handle forks on Group transfer to Organization

We need to figure out how to handle forks when we transfer top level groups to Organizations. The top level group and all subordinate entities will be moved to a new database. Forks will exist in a different Namespace to the top level group, which will be in a different database to the Organization.

In a comment, @mjwood said:

Briefly discussing with @andrashorvath and @jcaigitlab brought us to the preliminary conclusion that breaking forks will be very detrimental to our users and we're not really in favor of solutions where this is the overall plan. Technical issues (at first glance) would have us recommend that we avoid architectural changes where we will break forks.

However, to ensure we provide the best support to this effort as possible, we're planning on dedicating a resource (TBD) to dig deeper into this and to review the blueprint around cells and forking.

This issue exists as a place to formulate a plan to handle forks on group transfers.