FEATURE: Update topic/comment embedding parameters (#20181)

This commit implements many changes to topic and comments embedding. It
deprecates the class_name field from EmbeddableHost and suggests using
the className parameter. discourse_username parameter has been
deprecated and it will fetch it from embedded site from the author or
discourse-username meta.

See the updated code sample from Admin > Customize > Embedding page.

* FEATURE: Add className parameter for Discourse embed

* DEV: Hide class_name from EmbeddableHost

* DEV: Deprecate class_name field of EmbeddableHost

* FEATURE: Use either author or discourse-username meta tag

* DEV: Deprecate discourse_username parameter

* DEV: Improve embed code sample

app/assets/javascripts/admin/addon/controllers/admin-embedding.js

@@ -16,10 +16,14 @@ export default Controller.extend({
   @discourseComputed("embedding.base_url")
   embeddingCode(baseUrl) {
     const html = `<div id='discourse-comments'></div>
+<meta name='discourse-username' content='DISCOURSE_USERNAME'>
 
 <script type="text/javascript">
-  DiscourseEmbed = { discourseUrl: '${baseUrl}/',
-                     discourseEmbedUrl: 'REPLACE_ME' };
+  DiscourseEmbed = {
+    discourseUrl: '${baseUrl}/',
+    discourseEmbedUrl: 'EMBED_URL',
+    // className: 'CLASS_NAME',
+  };
 
   (function() {
     var d = document.createElement('script'); d.type = 'text/javascript'; d.async = true;