notifications: Support tapping a notification to open the conversation

[chrisbobbe]

After #122.

[chrisbobbe]

It might be useful to study how we do this in zulip-mobile. With

• ios [nfc]: Cut notifications library out of handling "initial notification" zulip-mobile#5740

, we simplify the case that was the most opaque in zulip-mobile, because it relied on a low-quality library: on iOS, tapping a notification to launch the app (from closed).