Share channels publicly

[aaronpk]

I created a channel for an event, and I want to be able to share it with others so they can also follow it.

This is pretty much what stream.indieweb.org is, so that's a good model to base it on.

Probably need a way to map a custom domain to a channel but also provide a url for channels so that it's faster.

When someone subscribes to a shared aperture channel from aperture, maybe bypass the feed fetching and copy objects internally to speed things up.

Also there is the question of whether a shared channel should be read only or if someone subscribed to one can remove entries out of it themselves.