Implement notification permission screen during onboarding

[GBKS]

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When first starting the application, it instantly asks for permission to send notifications. See the screenshot below.

There is no explanation as to why this permission is needed and the user has not even had a chance to try the application yet. But if they decline it, then the node will not run in the background. Let's add a permission info screen to the onboarding flow, right after we tell them we're not about to start the IBD. They will then have the proper context to make an informed decision. See the designs below.

[jarolrod]

cc @johnny9