admin.shopify.com related updates

[stlk]

Changes based on https://shopify.dev/apps/tools/app-bridge/updating-overview

• Migrate redirect calls to App Bridge 3.
• The template iframe_redirect.html now expects host parameter, not shop.
• session_tokens.views.get_scope_permission now detects embedded query parameter.
• session_tokens.views.FinalizeAuthView now redirects to the domain defined in host query parameter if present.