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

show and edit details for plan. Tinymce integrated with ES6. DMPRoadmap/roadmap#559

added generic layout partial for plan navigation tabs, bootstrapped the share plan page DMPRoadmap/roadmap#560