Fix up downloads URI and status handling.

This adds a pending status file ASAP after the request is accepted. It
also checks for the existence of such a file with status "PENDING"
before accepting another request for the same entity. If a request
exists it returns a 303 referring the caller to the in progress request.
Merge pull request #113 from MastodonC/feature/updates-to-download-status

Fix up downloads URI and status handling.
Init template with data.
Create separate template component.
Store checked boolean in local state.
Add downloads to the model.
Do not return 404 for uploads, return empty list instead.
Make download with data a button.
Return edn when queued download. Add alerts.
Rename uploads ns and add new route for download status.
Tidy up and prepare for entity subdirectories.
Tidy up UI
Fix subdirectories in uplaods/downloads.