Improvements to model access and label system

Including:

* Default to showing only a user's models on open
* Don't allow a model to be saved with an existing name (fixes #81 in csdms/wmt)
* Display an alert when a model is successfully deleted
* Don't allow users to delete other user's models! (client-side fix to #77 in csdms/wmt)
* Include owner and creation date in model metadata
* Overhaul label system to make code easier to read (and work correctly)
* Reorganize classes in the **ui** package