Push Notification Onboarding

Currently web-push (bundled with NodeBB) relies on the user to proactively find the "Push Notifications" sub-page from within the accounts page, and opt-in to push notifications.

Conversion rate: ~0.1% (the 0.1% is probably just me)

Let's improve that by prompting the user to enable push notifications on first invocation of the installed PWA

1. Listen for the appinstalled event and save a flag to local storage to trigger app onboarding flow
   • localStorage.setItem('pwa_onboarding', 'true');
2. Check for display-mode: standalone; (this is true when the installed app is opened, not the website) and the onboarding flag
   • const isPWA = window.matchMedia('(display-mode: standalone)').matches;
3. Redirect user to /app-onboard or similar, and allow plugins to inject text/urls into said page
4. Update web-push to inject a link to the push notification settings page.