ios notif: Control the showing of notifications

[gnprice]

When a notification-message comes in from the server on iOS, we should have some of our own Dart code run, and have that code control the notification that gets shown in the UI.

That's something we already do on Android: see NotificationDisplayManager. Most of the code of that class is specific to Android's notification API, so some refactoring and/or renaming will be needed.

For this issue, there doesn't need to be any change to how the notifications look or behave. If the notifications appear exactly the same way they did before, that's success.

One immediate way we can make use of having this control is to do the equivalent of #1264: check if the account is still logged in, and if not then suppress the notification.

Related issues

• An easy thing we can do as soon as we have this: the iOS equivalent of
  Don't show notifications for logged-out accounts, on Android #1264

• We can also use this control to improve the wording and formatting of notifications, similar to things we already do for Android notifications. (As far as possible, those changes should happen as separate commits or separate PRs, after the commit that resolves this issue.)

• Other issues this might help us with:
  notifications: Support "badge count" on iOS #153
  Show notifications when in foreground too, on iOS #408