DEV: Ignore bookmarks.topic_id column and remove references to it in code (#14289)

We don't need no stinkin' denormalization! This commit ignores
the topic_id column on bookmarks, to be deleted at a later date.
We don't really need this column and it's better to rely on the
post.topic_id as the canonical topic_id for bookmarks, then we
don't need to remember to update both columns if the bookmarked
post moves to another topic.

app/controllers/users_controller.rb

@@ -1598,7 +1598,10 @@ class UsersController < ApplicationController
         end
       end
       format.ics do
-        @bookmark_reminders = Bookmark.where(user_id: user.id).where.not(reminder_at: nil).joins(:topic)
+        @bookmark_reminders = Bookmark.with_reminders
+          .where(user_id: user.id)
+          .includes(:topic)
+          .order(:reminder_at)
       end
     end
   end