Add UI for team subscription

[wleese]

As mentioned in #330

We actually have a feature in-flight for this kind of use-case in Oncall. We call it "team subscription", which lets a team in Oncall borrow another team's events and put them onto its calendar. This lets you have TeamA "subscribe" to TeamB's primary events, and have them appear on TeamA's calendar. Then, when you query Oncall for TeamA's current primary, TeamB's primary will appear.

The main thing missing there is UI support; there's not yet a way to add them in the frontend. I think the calendar display for subscribed events is a little rough around the edges as well, but I haven't been able to get the time to throw that together. I think I'll check in with our web dev and our designer at LinkedIn and see what they're thinking about it.

After careful consideration, we'd not only like to use this feature, but also want to expose it to our users.

[wleese]

wrong project.