Refinements to vanish behaviour for Moderators and Admins.

For both types of staff:

 * Always vanish when entering ModMode.
 * Join silently if logged out vanished.

Moderators:

 * Keep vanish state (vanished or visible) across logins.

Admins:

 * Always vanish if joining the server in ModMode.
 * Keep vanish state across logins (vanished or visible) when not in ModMode.

The upshot of all this is:

 * Nobody needs to change their vanish state in order to be certain of being
   vanished when they enter ModMode. It will just "do the right thing".
 * Admins can signify that they want to join silently and vanished by logging
   out in their normal mode vanished, or by logging out in ModMode (vanished
   or not).  To show a log in message, leave ModMode and ensure you are
   unvanished before logging out.