Implement the "Attach another" button.

The initial input says "Attach Notice" and adds a file upload of kind 
"original". Additional inputs say "Supporting Document" and add file 
uploads of kind "supporting".

"original"s are not available except to admins (or possibly by specific 
request to researchers). "supporting" documents will be added to the 
show-notice page for download.

For now the integration specs are pending since we can't see the 
documents anywhere. I'm going to start a new branch for that. I may 
merge that one first so that I can rebase and finalize the integration 
specs here before merging this.

Note: once the types branch is merged, only my addition of the hidden 
input for file_upload[kind] will need to be propagated to the 
type-specific partials.