savePurposes() only checked that each key was a nonempty string, so a
stale or tampered mailbox key (reachable by posting straight to
/livewire/update) saved successfully while MailboxResolver silently
returned null for it. Every purpose's key must now name a mailbox that
exists; system's must additionally be active, since every unmapped or
inactive OTHER purpose falls through to it.
EditMailbox::save() also cleared last_verified_at only when the
password changed, but smtpUsername() depends on address and username
too (falling back to address when username is blank) — changing
either while leaving the password untouched kept showing a successful
verification for an identity nothing had actually tested.
3278f063ea