Allow creation of Iodide notebooks with files pre-attached via a POST request

[wlach]

#2676, when landed, should add the capability to create an iodide notebook from a template just by passing an iomd parameter to the get request. This is a good start, but doesn't handle the (probably common) case where the user might want to attach a file or two to the notebook.

The amount of content we can put in a URL caps out at varying lengths depending on your client/server (https://stackoverflow.com/questions/417142/what-is-the-maximum-length-of-a-url-in-different-browsers), so I think we'd want to attach these files in a form in a post request. I discussed this in this comment on #2565:

#2565 (comment)

Note that the POST request will be unauthenticated, so log in / authentication / saving would have to occur (if applicable) after the notebook (along with its files) is loaded. For that to work, we would have to also fix the (just filed) #2680.