[Dev] Subscribing to calendar URLs

[Grant Baillie]

On IRC earlier today, we had a user, Leppy, unable to subscribe to  
his Basecamp calendars (see <http://basecamphq.com/>) from Chandler;  
both Mozilla and Apple iCal both had no problems.

What's going on here is that Chandler is getting itself confused  
about what kind of thing you're trying to subscribe to. It could be  
any of:

   1. A simple text/calendar resource (i.e. a ".ics file on the web").
   2. A non-CalDAV Chandler WebDAV collection (i.e. cloud xml)
   3. A CalDAV collection

It turns out that we don't really detect the first case very well; in  
fact, if you're not subscribing to a URL that ends in ".ics",  
Chandler won't allow you to subscribe, and will fail with a fairly  
incomprehensible error.

While I'm working on making this code work better (i.e. correctly) in  
0.7, it seemed to me that we could make a simple change in the  
sharing code for 0.6.1 that would enable more users to subscribe to  
shared calendars:

Basically, if the URL being subscribed to is using the 'webcal'  
scheme, we should assume case #1 above. This would fix the user's  
problem, at least, and others where calendars get shared with a  
webcal: scheme but no .ics extension.

FWIW, the webcal scheme is not an official standard; rather I think  
it's something Apple made up, but has become fairly widespread nowadays.

Any comments / suggestions?

--Grant