Set basic server-side notification settings

[gnprice]

We should support the three user settings enable_offline_push_notifications, enable_online_push_notifications, enable_stream_push_notifications. These let the user control, at a coarse granularity, whether and when the server sends them push notifications.

The user docs for these settings are here:
https://zulip.com/help/mobile-notifications
https://zulip.com/help/stream-notifications
In particular these three are supported in the legacy zulip-mobile app.

In the API, they appear in the same three places (for initial data, events, and user requests to edit them) as all user settings. See the existing settings in our code listed in the UserSettingsName enum, and look at the references to any one of those enum values.

[chrisbobbe]

This came up in beta feedback today:

As I enabled notifications for Flutter beta and allowed everything on OS/Android level. And at the end came an idea that I disabled them via web app.

[gnprice]

For cross-reference: there's a separate feature for setting the per-channel notification settings. That's tracked as part of this other issue:

• UI for channel subscription settings other than muted/unmuted #1223