Bug 976804: Implement new upgrade banners.

Includes a management command for generating a .htaccess file for the
media directory, that controls the display of the banner images.

Upgrade banner image display won't work unless you have apache serving
up the media directory instead of Django as it relies on the .htaccess
file.
Empty out product_details and add .gitignore for it.
Include support for legacy upgrade banners by explicitly including them.
Remove storage tests (too simple) and add tests for upgrade banners.
Make update_product_details able to run when json directory is empty.

Removes dependence on product_details to load settings and models. Also
adds a note to the install docs to download the product_details JSON
files.