Checked items now work

This address issue #118.  The download and delete actions work as expected.  Internally, there are two new endpoints: resource/download GET and resource DELETE.  The download can actually get item, folder, user, and collection resources, and the delete can work on any model that has a remove method.
The way we use this function, we end up loading all of the models twice, but since this doesn't truly guarantee that the task will be valid through its full duration, I think we should save the time and code and just get rid of this. If the client passes an invalid model name or an invalid model ID, I'm OK with the task deleting some of the selected resources then erroring out partway through.