Add string versions of User/Pickled objects to datastore models

On newly created objects Package.uploaders,
PackageVersion.uploader/readme/changelog will have corresponding String
variants.

After a batch processing update of old models, we can enforce the usage of the
new properties (and drop the old ones).

R=sgjesse@google.com

Review URL: https://chromiumcodereview.appspot.com//816693002