Support 365 mailbox instead of dovecot for incoming mails

• Support IMAP OAUTH authentication for 365. Basic support is not available.
  • details https://mailservices.docs.cern.ch/ExchangeOnline/programmatic/
  • code sample https://gitlab.cern.ch/mail/mailservices-code-samples
  • will need a persistent storage to store the token for refesh
  • needs to be discussed for first call
    • interactive browser required: To sign in, use a web browser to open the page https://microsoft.com/devicelogin and enter the code XXXXX to authenticate.
• Support only ONE + in the address, 365 fails with two !
  • notifications+<channel-name-slug>+<level>@dovecotmta.cern.ch will not work anymore
    • option 1: replace second + by another char
    • option 2: drop +<level> functionnality and keep only notifications+<channel-name-slug>@cern.ch
    • other options ?