Reorganise zone status

[tertsdiepraam]

The status field in zone status will have 7 steps after #901. This is a bit much, so we might need to reorganise it a bit. For example:

status: signing
  ✔ load (serial: 1776081352)
  ● sign
  ├─╮
  | ○ sign the loaded zone
  | |   loaded serial: 1776081352
  | |   signed serial: <not yet known>
  | |   start time: <not started yet>
  | |
  | ○ review signed zone (disabled)
  | ○ persist signed zone
  ├─╯
  ○ publish

This would hide the steps outside of the current loading or signing operation and simplify the view. --detailed could still show everything.