more info in storage README and function comments
Have @storage_required look in MEDIA_BASE, not storage_required.

We have been seeing error messages when the symlink in MEDIA_BASE
is not present:
    "/var/www/media/base does not exist or is not a directory"

The @storage_required decorator was telling the collections_local
function to look for collections in storage.base_path, while the
rest of the DDR was using MEDIA_BASE.
storage.ddr_media_dir really is MEDIA_BASE, admit
rm old media symlink before adding new one, just in case
make sure there's a redirect url before redirecting to it
make pre-remount URL a clickable link, even if it's still hidden
added more helpful and reassuring help text