Use DNS SRV records to get CalDAV information

[malini]

Is your feature request related to a problem? Please describe.
Users have to find their own calendar urls right now, which is tedious and a different workflow for whatever service you're using, so instead of having the user provide their CalDAV urls, we should try to retrieve that information for them given the user's email address.

Describe the solution you'd like
Instead of having the user provide their CalDAV urls, we should try to retrieve that information for them given the user's email address.

[MelissaAutumn]

I'm tying this into the existing issue as its pretty easy.

[MelissaAutumn]

Part of ##718