Use DataManager#findModel in DeleteModelHandler

Fixes a bug where a model that had been owned by someone other than the
user wasn’t getting deleted.

The findModel method has been really helpful.
Return null if owner, date undefined in ModelListJSO

Update unit tests, as well.
Show "read-only" in title of public model

To be clear, a public model that the current user doesn’t own.
Merge pull request #42 from mdpiper/master

Show "read-only" in title of public model not owned by current user