DKIM for base domain

[lpoujade]

Describe the bug

AgentJ web URL can, and often is, different than the mail domain. Default no-reply address use the web domain so in a classic setup it doesn't match the configured DKIM key which is only generated for manually created domains.
The result is validation mails which a bad DKIM signature

To Reproduce

• install AgentJ on a different domain than the mail domain (eg web.domain.coop if mail domain is domain.coop)
• create a domain and keep the default no-reply mail (no-reply@web.domain.coop)
• send a mail to a protected email with an external server
• examine DKIM signature of validation mail

Expected behavior

All mails sent from/via AgentJ must have a correct DKIM signature

• when an instance is started for the first time, a DKIM key should be generated
• corresponding public key should by local and super admins (with a DNS-ready format)
• maybe we should be able to regenerate this key (only super admin)
• maybe there should be a form somewhere to generate key from whatever domain we need (by super admin: visible for everyone, by local admin: only in their domain)

Additional context