chore(rethinkdb): Comment: Phase 2

[mattkrick]

Description

migrates existing rows

NOTE:
during phase 1 we encountered a severe bug in prod. This was initially fixed by manually removing a FK constraint in PG and then hotfixed in #10178.

I don't like manually adjusting the PG schema in realtime, but the bug was REALLY bad and by removing that constraint live, we were able to get meetings working again in a second vs 15 minutes.

Before merging this, I will readd the following constraint:

CONSTRAINT "fk_discussionId"
        FOREIGN KEY("discussionId")
          REFERENCES "Discussion"("id")
          ON DELETE CASCADE