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

spec/requests/admin/embeddable_hosts_controller_spec.rb

@@ -56,7 +56,6 @@ RSpec.describe Admin::EmbeddableHostsController do
             params: {
               embeddable_host: {
                 host: "test.com",
-                class_name: "test-class",
                 category_id: category.id,
               },
             }
@@ -67,7 +66,7 @@ RSpec.describe Admin::EmbeddableHostsController do
           UserHistory.where(
             acting_user_id: admin.id,
             action: UserHistory.actions[:embeddable_host_update],
-            new_value: "category_id: #{category.id}, class_name: test-class, host: test.com",
+            new_value: "category_id: #{category.id}, host: test.com",
           ).exists?
 
         expect(history_exists).to eq(true)
@@ -82,7 +81,6 @@ RSpec.describe Admin::EmbeddableHostsController do
             params: {
               embeddable_host: {
                 host: "test.com",
-                class_name: "test-class",
                 category_id: category.id,
               },
             }