This will let us do an `else if`.

Following the generic "server event" pattern Greg started in
979d283. We still convert avatar_url to our special AvatarURL form
at the edge, in eventToAction, for crunchy shell:
  https://github.com/zulip/zulip-mobile/blob/master/docs/architecture/crunchy-shell.md

Also, give a Flow type to the event itself as we receive it from the
server: RealmUserUpdateEventRaw. We should keep this up-to-date with
the server API documentation. (We also have RealmUserUpdateEvent,
for the slightly processed form that has AvatarURL.)

We still keep the behavior of only allowing avatar_url and role
changes to apply to the state [1]. But now, that code is in
usersReducer instead of eventToAction. For the reducer tests that
simulate actions changing *other* user data, that means they'd now
fail. So, skip them, with a plan to re-enable.

[1] Because we've been wary of accepting server data that we haven't
    vetted; see 7296551. The API doc seems to have been improved
    since then, and now we've written down a type for it. So now we
    can proceed more confidently, which is nice!

The reducer still doesn't handle all these cases, so we can't
un-skip them yet. But now at least we're prepared to cover all the
cases once the reducer *does* handle them, soon.

See the list of cases in the doc:
  https://zulip.com/api/get-events#realm_user-update

The removed comment was wrong to say that we should exclude a test
for bot-owner changes. The users state doesn't include cross-realm
bots, it's true. But it does include regular bots, and those are the
bots that have owners.

Replaces: zulip#3403

See zulip/zulip#22103, which I sent after
observing that this was sometimes null.