Calendar Support

# Use case

Calendar support for WebDAV, CalDAV and CardDAV.

What problem you are trying to solve?

A monolithic integration of calendar service and mail. The calendar then uses the same credentials of the mail users.

How your solution would work in general?

Use go-webdev package to provide calendar service: https://github.com/emersion/go-webdav

A new thread serves the calendar along with mail service on other threads. Sharing the same credentials and facilitating the usage of both services is the key.