[keyserver] Introduce migration to regenerate KNOW_OF relationships

Summary:
As a result of running the recent migration with, we inadvertently created KNOW_OF relationships between each pair of users.

This occurred because each user got a memberships row in each thread in GENESIS, even though they were not a member. That's because their member_ permissions got cascaded down to all threads because of D13113.

To get around this, I deleted all KNOW_OF relationships in production, and regenerated them using this migration.

More details in ENG-9075.

Test Plan: I ran this in production. Before I ran it there were 563736 KNOW_OF relationships. After running it there were only 31617

Reviewers: will, varun

Reviewed By: will

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13144