Add support for webapp's new configurable notification sounds feature

[timabbott]

Via zulip/zulip#8061, we recently added Zulip webapp support for picking a notification sound from a set of (currently) 2 choices. Before doing mobile work on this, we should wait for the server/client API to work how we want it to (right now, we send just a name, e.g. "ding", to tell a client what sound to use, and I think we should instead send a dictionary structure mapping available sound file formats to full sound URLs, as described in zulip/zulip#8061 (comment)), but I'm opening the mobile issue now so that we don't forget about it.

[borisyankov]

I suspect this will be more complicated on mobile than it sounds to be useful if implemented fully flexibly like that.
What if we just supply the files up front and the only option given is one of the names?

[timabbott]

Well, we can certainly do that for v1, and plan to extend with "if the key is a URL, fetch it".