FEATURE: Native app banner improvements

This commit adds some improvements to native app banners for iOS and Android

- iOS and Android now have separate settings for native app banners

- app banners will now only show for users on TL1 and up

- app ids are now in a hidden site setting to allow sites to switch to their own app, if desired

- iOS only: the site URL is passed to the app arguments

db/migrate/20190417135049_migrate_native_app_banner_site_setting.rb

@@ -0,0 +1,20 @@
+class MigrateNativeAppBannerSiteSetting < ActiveRecord::Migration[5.2]
+  def up
+    execute "INSERT INTO site_settings(name, data_type, value, created_at, updated_at)
+             SELECT 'native_app_install_banner_android', 5, value, now(), now()
+             FROM site_settings
+             WHERE name = 'native_app_install_banner'"
+
+    execute "UPDATE site_settings
+             SET name = 'native_app_install_banner_ios'
+             WHERE name = 'native_app_install_banner'"
+  end
+
+  def down
+    execute "UPDATE site_settings
+             SET name = 'native_app_install_banner'
+             WHERE name = 'native_app_install_banner_ios'"
+
+    execute "DELETE FROM site_settings WHERE name = 'native_app_install_banner_android'"
+  end
+end