Manage MX "half installed" for migrations & altospam-like services

[vincib]

In AlternC, we can only say "yes" or "no" in "do we manage emails for this domain".

• The "no" destroys mailboxes and disallow to edit mailboxes
• the "yes" allow to edit mailboxes AND tell postfix that we handle this domain.

We should have a 3-state instead :

• we manage the mail locally entirely
• we don't manage the mail locally at all
• we manage mailboxes but postfix is not the MX (imap ok, but smtp uses the DNS-published MX) this state could be used to facilitate migrations

IMPORTANT: in the 3rd state we should put a RED warning at the top of the "domain" page.

[vincib]

proposal from arnaud : adds a flag "gesmailbox" to allow mail management in the menu

• only used by "menu_mail" to show or hide a domain
• warning: need to check that postfix has "AND gesmx=1" in all queries