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.

app/serializers/post_serializer.rb

@@ -367,10 +367,18 @@ class PostSerializer < BasicPostSerializer
   end
 
   def post_bookmark
-    if @topic_view.present?
-      @post_bookmark ||= @topic_view.user_post_bookmarks.find { |bookmark| bookmark.post_id == object.id && !bookmark.for_topic }
+    if SiteSetting.use_polymorphic_bookmarks
+      if @topic_view.present?
+        @post_bookmark ||= @topic_view.bookmarks.find { |bookmark| bookmark.bookmarkable == object }
+      else
+        @post_bookmark ||= Bookmark.find_by(user: scope.user, bookmarkable: object)
+      end
     else
-      @post_bookmark ||= object.bookmarks.find_by(user: scope.user, for_topic: false)
+      if @topic_view.present?
+        @post_bookmark ||= @topic_view.bookmarks.find { |bookmark| bookmark.post_id == object.id && !bookmark.for_topic }
+      else
+        @post_bookmark ||= object.bookmarks.find_by(user: scope.user, for_topic: false)
+      end
     end
   end