Add persisting states to the state machine and zone status

[tertsdiepraam]

Looks like this in zone status:

status: persisting loaded zone
  ✔ load (serial: 1776081352)
  ○ review loaded zone (disabled)
  ● persist loaded zone
  |
  ○ sign
  ○ review signed zone (disabled)
  ○ persist signed zone
  ○ publish

I think this closes #867, at least as far as I can tell from the description.