Add support for Office 365 modern authentication.

For each post box, add an authentication type to the configuration; options are basic and O365.
Basic requires email and password, modern only username.

When starting the app for the first time, a token has to be retrieved. The app should go through each O365 post box and start an authentication flow.

To make sure the tokens are not lost, securely store them.

Lastly adapt the mail fetching.