DEV: update index.html files (#22695)

This brings them more in line with an idiomatic ember app looks
like, in particular, embroider really expects the CSS file to be
there.

As far as I can tell this is fairly harmless, since in production
the actual HTML is generated and served by Rails anyway.

Down the road, this may also be a good alternative to hacking the
build pipeline to bring in styles for tests.

app/assets/javascripts/discourse/app/index.html

@@ -19,6 +19,9 @@
     <bootstrap-content key="discourse-preload-stylesheets">
     {{content-for "discourse-preload-stylesheets"}}
 
+    <link integrity="" rel="stylesheet" href="{{rootURL}}assets/vendor.css" />
+    <link integrity="" rel="stylesheet" href="{{rootURL}}assets/discourse.css" />
+
     <script defer src="{{rootURL}}assets/vendor.js"></script>
     <script defer src="{{rootURL}}assets/discourse.js"></script>
 

app/assets/javascripts/discourse/app/styles/app.css

@@ -0,0 +1,4 @@
+/**
+ * 👋 Greetings Discourse Developer. This CSS file is unused. If you're looking
+ * for stylesheets managed by Rails, see app/assets/stylesheets at project root.
+ */

app/assets/javascripts/discourse/tests/index.html

@@ -18,6 +18,7 @@
     {{content-for "head"}} {{content-for "test-head"}}
 
     <link rel="stylesheet" href="{{rootURL}}assets/vendor.css" />
+    <link rel="stylesheet" href="{{rootURL}}assets/discourse.css">
     <link rel="stylesheet" href="{{rootURL}}assets/test-support.css" />
     <link rel="stylesheet" href="{{rootURL}}assets/qunit.css" />
     <link rel="stylesheet" href="{{rootURL}}assets/qunit-custom.css" />