DEV: Make Bookmark#post_id column nullable (#16287)

As we are gradually moving to having a polymorphic
bookmarkable relationship on the Bookmark table,
we need to make the post_id column nullable to be
able to develop and test the new columns, and
for cutover/migration purposes later as well.

app/models/bookmark.rb

@@ -169,7 +169,7 @@ end
 #
 #  id                     :bigint           not null, primary key
 #  user_id                :bigint           not null
-#  post_id                :bigint           not null
+#  post_id                :bigint
 #  name                   :string(100)
 #  reminder_at            :datetime
 #  created_at             :datetime         not null