Allow editing calendar url

[cyberarm]

I subscribe to some calendars that periodically invalidate the url token, e.g. https://example.com/forum/calendar/2-meetings/download/?key=<token>, it'd be nice if I could just update the url instead of needing to delete and recreate the calendar to get an updated url in.

[ArnyminerZ]

Mmmmh... Such an specific use-case, but I don't see why this couldn't be done. What do you think @rfc2822 ?

[rfc2822]

Don't know about the UI and whether it could confuse people or so, but otherwise there shouldn't be a problem with editing the URL

[ArnyminerZ]

Maybe we can create a sub-screen or something, for not overloading the main edit screen. How about waiting until #91 so we can further discuss the design changes?