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.

db/post_migrate/20210909041448_make_topic_id_nullable_for_bookmarks.rb

@@ -0,0 +1,13 @@
+# frozen_string_literal: true
+
+class MakeTopicIdNullableForBookmarks < ActiveRecord::Migration[6.1]
+  def up
+    change_column_null :bookmarks, :topic_id, true
+    Migration::ColumnDropper.mark_readonly(:bookmarks, :topic_id)
+  end
+
+  def down
+    Migration::ColumnDropper.drop_readonly(:bookmarks, :topic_id)
+    change_column_null :bookmarks, :topic_id, false
+  end
+end