Add support for T-Online - CALDAV implementation (Deutsche Telekom Calendar service)

Hi all,

yesterday I've installed your App for the first time.
Everything looks fine and clearly designed, but I've one problem with the CALDAV implementation.

I use OneCalendar for synchronization with a Deutsche Telekom (T-Online) email account (@t-online.de).

The CALDAV URL is ....
https://spica.t-online.de/spica-calendar/caldav/

The problem:
Everytime I created a new calendar entry, the Huawei phone synchronize, and after 3 seconds I get a second calendar entry with identical data.

It looks like the Caldav server is doing something different from the other caldav servers you have implemented before.

It would be great if OneCalendar would have an integration to T-Online in the near future.

Many hanks for that!

Rgds
Holger