Add ability to download CAD files from detail pages

[xmedr]

Overview

CAD files for FlatDrawings were only downloadable from that file's edit page. This change lists the CAD file as a direct download link of the detail page.

Closes #82

Demo

Testing Instructions

• Navigate to a flatdrawing detail page like http://localhost:8000/flatdrawings/1395/
  • If it doesn't have a CAD file, upload a dummy file in the edit page
• Click the link next to the CAD label and confirm that the file downloads
• Also, confirm that pages without CAD files still load without error

[xmedr]

@smcalilly Ah cool, that's useful!