Bootstrapping and ES6 of the Plan Download page DMPRoadmap/roadmap#561. ...

- Renamed show_export.html.erb to download.html.erb (updated controller, policy, links and route)
- Renamed export_settings.js to download.js and dropped unused auto-save JS (no longer a part of wireframes)
- Bootstrapped page html

changed name of show_export to download to match UI terminology. Bootstrapped download page

Added ES6 JS for download page and made tweaks to the format