Fix loading files attached to plugin models
Fix loading files attached to plugin models

Plugins may attach files to a model type defined in the plugin. In that
case, the "attachedToType" field is a list of the form ["model_type",
"plugin_name"].

This commit fixes errors like the following when loading such files:

    {
        "TypeError: TypeError("unhashable type: 'list'",)",
        "trace": [
            ...
            [
                "/path/to/girder/girder/utility/acl_mixin.py",
                70,
                "load",
                "self.model(loadType).load(loadId, level=level, user=user, exc=exc)"
            ],
            [
                "/path/to/girder/girder/utility/model_importer.py",
                88,
                "model",
                "if model not in _modelInstances[plugin]:"
            ]
        ],
        "type": "internal"
    }

jbeezley/girder,Xarthisius/girder,Xarthisius/girder,girder/girder,kotfic/girder,data-exp-lab/girder,kotfic/girder,manthey/girder,Xarthisius/girder,Kitware/girder,data-exp-lab/girder,kotfic/girder,girder/girder,jbeezley/girder,Xarthisius/girder,RafaelPalomar/girder,manthey/girder,kotfic/girder,kotfic/girder,RafaelPalomar/girder,girder/girder,RafaelPalomar/girder,manthey/girder,Kitware/girder,Kitware/girder,girder/girder,jbeezley/girder,data-exp-lab/girder,Kitware/girder,RafaelPalomar/girder,jbeezley/girder,manthey/girder,data-exp-lab/girder,data-exp-lab/girder,Xarthisius/girder,RafaelPalomar/girder