Make sure we have a plan for resolving seniority when two users were added concurrently

[HerbCaudill]

In the bySeniority sort function, we just check if a is a predecessor of b and if not, we assume that b is a predecessor of a. But they could have been added concurrently, in which case I'm not sure the current behavior is deterministic.

Test plan:

• Two users are admitted concurrently.
• They're both promoted to admins.
• They try to remove each other concurrently.
• Assert that everyone on the team resolves the conflict the same way