Add mailbox.org DAV provider

See these help center articles:

* https://kb.mailbox.org/en/private/calendar-article/caldav-and-carddav-for-other-devices
* https://kb.mailbox.org/en/private/calendar-article/how-to-embed-calendars-in-korganizer

Note that this change is completely untested and not compiled. I just copied and changed the similar `yahoo.desktop` file.