* Add CardDav and CalDav servers
This implements the servers, but not the clients (yet). The implementation is essentially a proxy to Radicale (as a container in `docker-compose.yml`), with certain security assurances.
If you're upgrading, basically you'll need to create a new `data-radicale` directory, and everything else should just work.
This will also release v2.3.0 with those enabled by default. Tested with Thunderbird and Apple Calendar + Contacts.
To disable these, simply add the new config details and comment out or don't add the new `radicale` service from `docker-compose.yml`.
Related to #56
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