Turn off non-critical notifications on mobile

When turning of notifications on mobile, text appears that reads:

> You turned off native notifications for keybase. It's _very_ important you turn them back on.

However, the app does not give the ability to distinguish between notifications that are "very important", which I assume would be new device/key change/etc notifications, and other notifications, like normal chat and team messages.

If you would like users to opt in to receiving critical notifications, and thus be able to use the app safely, you should allow them to turn off non-critical notifications, otherwise they will, like me, turn off notifications entirely.

Locate the mobile notification settings and notification-handling entry points, then determine how native notifications are currently enabled or disabled. Done should allow critical and non-critical notifications to be controlled separately and provide wording that matches the selected state.

Written by the indexing model from the issue text.