FEATURE: Promote polymorphic bookmarks to default and migrate (#16729)

This commit migrates all bookmarks to be polymorphic (using the
bookmarkable_id and bookmarkable_type) columns. It also deletes
all the old code guarded behind the use_polymorphic_bookmarks setting
and changes that setting to true for all sites and by default for
the sake of plugins.

No data is deleted in the migrations, the old post_id and for_topic
columns for bookmarks will be dropped later on.

app/controllers/users_controller.rb

@@ -1725,20 +1725,13 @@ class UsersController < ApplicationController
         end
       end
       format.ics do
-        if SiteSetting.use_polymorphic_bookmarks
-          @bookmark_reminders = Bookmark.with_reminders
-            .where(user_id: user.id)
-            .order(:reminder_at)
-            .map do |bookmark|
-            bookmark.registered_bookmarkable.serializer.new(
-              bookmark, scope: user_guardian, root: false
-            )
-          end
-        else
-          @bookmark_reminders = Bookmark.with_reminders
-            .where(user_id: user.id)
-            .includes(:topic)
-            .order(:reminder_at)
+        @bookmark_reminders = Bookmark.with_reminders
+          .where(user_id: user.id)
+          .order(:reminder_at)
+          .map do |bookmark|
+          bookmark.registered_bookmarkable.serializer.new(
+            bookmark, scope: user_guardian, root: false
+          )
         end
       end
     end