FEATURE: Polymorphic bookmarks pt. 1 (CRUD) (#16308)

This commit introduces a new use_polymorphic_bookmarks site setting
that is default false and hidden, that will be used to help continuous
development of polymorphic bookmarks. This setting **should not** be
enabled anywhere in production yet, it is purely for local development.

This commit uses the setting to enable create/update/delete actions
for polymorphic bookmarks on the server and client side. The bookmark
interactions on topics/posts are all usable. Listing, searching,
sending bookmark reminders, and other edge cases will be handled
in subsequent PRs.

Comprehensive UI tests will be added in the final PR -- we already
have them for regular bookmarks, so it will just be a matter of
changing them to be for polymorphic bookmarks.

lib/topic_view.rb

@@ -413,9 +413,15 @@ class TopicView
   end
 
   def bookmarks
-    @bookmarks ||= @topic.bookmarks.where(user: @user).joins(:topic).select(
-      :id, :post_id, "topics.id AS topic_id", :for_topic, :reminder_at, :name, :auto_delete_preference
-    )
+    if SiteSetting.use_polymorphic_bookmarks
+      @bookmarks ||= Bookmark.for_user_in_topic(@user, @topic.id).select(
+        :id, :bookmarkable_id, :bookmarkable_type, :reminder_at, :name, :auto_delete_preference
+      )
+    else
+      @bookmarks ||= @topic.bookmarks.where(user: @user).joins(:topic).select(
+        :id, :post_id, "topics.id AS topic_id", :for_topic, :reminder_at, :name, :auto_delete_preference
+      )
+    end
   end
 
   MAX_PARTICIPANTS = 24
@@ -512,10 +518,6 @@ class TopicView
     @links ||= TopicLink.topic_map(@guardian, @topic.id)
   end
 
-  def user_post_bookmarks
-    @user_post_bookmarks ||= @topic.bookmarks.where(user: @user)
-  end
-
   def reviewable_counts
     @reviewable_counts ||= begin
       sql = <<~SQL