Pulling in calendar feeds

Would be great if we could subscribe our feed to the iCal feed of other orgs, at least to have to review if not to post directly to the cal. Like, MIT Civic Media, New Urban Mechanics, etc.

The issue names no files, tests, or entry points. Start by defining how iCal feeds from other organizations should be subscribed to and whether imported events require review before posting; done should include an agreed workflow for feeds such as MIT Civic Media and New Urban Mechanics.

Written by the indexing model from the issue text.