R1:f686a0b8275e

In Phortune accounts, prevent self-removal more narrowly

Summary:
Currently, Phortune attempts to prevent users from removing themselves as account managers. It does this by checking that the new list includes them.

Usually this is sufficient, because you can't normally edit an account unless you're already a manager. However, we get the wrong result (incorrect rejection of the edit) if the actor is omnipotent and the acting user was not already a member.

It's okay to edit an account into a state which…