s/TastyPie/Tastypie/

Misspelling ``storybase.collections.TastypieMixin`` as
``storybase.collections.TastyPieMixin`` caused the API response to be
improperly parsed (specifically it didn't search inside the ``objects``
property of the response JSON) which in turn resulted in asset type
restrictions and contextual help stored in the container template models
to be reflected in the builder UI.

Closes #763