DEV: Remove OpenComposer mixin and refactor related logic (#23015)

The OpenComposer mixin comes from a time before we had a composer service. As well as being a general cleanup/refactor, this commit aims to removes interlinking between composer APIs and the discovery-related controllers which are being removed as part of #22622.

In summary, this commit:
- Removes OpenComposer mixin
- Adds and updates composer service APIs to support everything that `openComposer` did
- Updates consumers to call the composer service directly, instead of relying on the mixin (either directly, or via a route-action which bubbled up to some parent)
- Deprecates composer-related methods on `DiscourseRoute` and on the application route

spec/system/composer/default_to_subcategory_spec.rb

@@ -41,13 +41,9 @@ describe "Default to Subcategory when parent Category doesn't allow posting", ty
             end
           end
           describe "Category does not have subcategory" do
-            it "should have the 'New Topic' button enabled and default Subcategory set to latest default subcategory" do
+            it "should have the 'New Topic' button disabled" do
               category_page.visit(category_with_no_subcategory)
-              expect(category_page).to have_button("New Topic", disabled: false)
-              category_page.new_topic_button.click
-              select_kit =
-                PageObjects::Components::SelectKit.new("#reply-control.open .category-chooser")
-              expect(select_kit).to have_selected_value(default_latest_category.id)
+              expect(category_page).to have_button("New Topic", disabled: true)
             end
           end
         end
@@ -70,13 +66,9 @@ describe "Default to Subcategory when parent Category doesn't allow posting", ty
         end
         describe "Can't post on parent category" do
           describe "Category does not have subcategory" do
-            it "should have the 'New Topic' button enabled and default Subcategory not set" do
+            it "should have the 'New Topic' button disabled" do
               category_page.visit(category_with_no_subcategory)
-              expect(category_page).to have_button("New Topic", disabled: false)
-              category_page.new_topic_button.click
-              select_kit =
-                PageObjects::Components::SelectKit.new("#reply-control.open .category-chooser")
-              expect(select_kit).to have_selected_name("category…")
+              expect(category_page).to have_button("New Topic", disabled: true)
             end
           end
         end