FIX: only use app argument for official iOS app banner

When showing the native app banner, we include an app argument to automatically add the current site to the official DiscourseHub app. However, the app id can be changed via a hidden site setting, and when changed, that argument is no longer useful. This ensures the argument is only included for the official iOS app banner.

app/helpers/application_helper.rb

@@ -338,6 +338,12 @@ module ApplicationHelper
     current_user && current_user.trust_level >= 1 && SiteSetting.native_app_install_banner_ios
   end
 
+  def ios_app_argument
+    # argument only makes sense for DiscourseHub app
+    SiteSetting.ios_app_id == "1173672076" ?
+      ", app-argument=discourse://new?siteUrl=#{Discourse.base_url}" : ""
+  end
+
   def allow_plugins?
     !request.env[ApplicationController::NO_PLUGINS]
   end

app/views/layouts/application.html.erb

@@ -44,7 +44,7 @@
     <link rel="manifest" href="<%= Discourse.base_uri %>/manifest.webmanifest" crossorigin="use-credentials">
 
     <%- if include_ios_native_app_banner? %>
-        <meta name="apple-itunes-app" content="app-id=<%= SiteSetting.ios_app_id %>, app-argument=discourse://new?siteUrl=<%= Discourse.base_url %>">
+        <meta name="apple-itunes-app" content="app-id=<%= SiteSetting.ios_app_id %><%= ios_app_argument %>">
     <%- end %>
 
     <%= render partial: "common/discourse_stylesheet" %>