The identity's signature will be configured according to signature.text
and signature.showSignature. Signature commands are not supported by
Thunderbird.
The added extraConfig option allowes users to insert custom text at
the end of the generated profile `user.js` file. This allows the users
to import templates as part of their configuration.
Add a new Thunderbird module that uses the configuration in
`accounts.email.accounts` to setup SMTP and IMAP accounts.
Multiple profiles are not supported at this point.
