- introduce cyclically dependent effects: UserSubsystem, AuthenticationSubsystem (see Brig.CanonicalInterpreter).
- introduce TeamInvitationSubsystem with operations inviteUser, internalCreateInvitation.
- add verifyPassword to AuthenticationSubsystem.
- add sendInvitationMail, sendInvitationMailPersonalUser to EmailSubsystem.
- add getTeamSize to IndexedUserStore (this is morally internal to wire-subsystems, and making another ES subsystem would mean adding a lot of code everywhere).
- add updateUserTeam to UserStore.
- add acceptTeamInvitation, internalFindTeamInvitation to UserSubsystem.
- make a few small rest api handlers in brig polysemic (Handler -> Sem).