uploads: mark old columns as ignored.

Mark old columns as ignored in preparation for dropping them. Make the
rating and tag_string nullable so they don't have to be set when
creating uploads and can be ignored too.

db/migrate/20220203040648_drop_not_null_constraints_from_uploads.rb

@@ -0,0 +1,6 @@
+class DropNotNullConstraintsFromUploads < ActiveRecord::Migration[7.0]
+  def change
+    change_column_null(:uploads, :rating, true)
+    change_column_null(:uploads, :tag_string, true)
+  end
+end