Link to system notification settings

[gnprice]

When a user wants to change whether Zulip's notifications make a sound or not, whether they pop up visibly on the screen, and so on, the way to do that is in a certain place in the system settings app — both iOS and Android bar the app from controlling that, so we can't have our own settings UI for it.

Those settings can be hard to find, though, especially on Android, and I think most users aren't familiar with them. So:

What I have seen some apps do is they'll have an item in their settings UI that is basically a link to the relevant page of the system settings — you tap it and you get taken to that page.

We have one of those in the legacy app. We should add one in the Flutter app too, once we build out a bit of a settings UI in the first place.

Related issues

Two other issues call for other features of the legacy app's notification settings UI:

• Warn when haven't successfully sent notification token to server #323
• Set basic server-side notification settings #661

[gnprice]

Ideally perhaps we can even have it go straight to the relevant notification channel, on Android where that distinction exists. That would help users bypass one of the ways that Android's handling of this is confusing, as seen in a chat thread today (part 1; part 2).