[MU] Make impersonification work with Mail clients

[chibenwa]

What

The user authenticates and access an account other than his.

# Correct user with correct password can authenticate with any delegation part
C: 0007 AUTHENTICATE "PLAIN" {36+}
# delegate\0imapuser\0password
C: ZGVsZWdhdGUAaW1hcHVzZXIAcGFzc3dvcmQ=
S: 0007 OK AUTHENTICATE completed.

This can be used to leverage delegation (this is what GCS sara does).

Work to be done

Identify a mail user agent (Ideally thunderbird or Outlook 2013) that supports impersonation.

[Arsnael]

Timeboxed to 1d

[chibenwa]

Abandonned in favor of https://github.com/linagora/tmail-backend/issues/1132