use MEDIA_BASE in ddrlocal,webui rather than storage.base_path()

- storage.base_path() ("/media/WD5000BMV-2/ddr") points to the
  actual filesystem path of the DDR repo, which is a USB HDD.
- settings.MEDIA_ROOT is "/var/www/media".
- settings.MEDIA_BASE ("/var/www/media/base") is a symlink to
  storage.base_path().
- settings.MEDIA_ROOT is not something that can be easily changed
  during runtime.  Nginx's "location /media/" is also not easily
  changed during runtime.

- When loading collections,entities, we have been using
  base_path() rather than MEDIA_ROOT.
- ddrlocal file manipulations have been outside the scope of
  Django so this looked OK.

- sorl.thumbnail has also been using base_path(), and uses
  Django's filesystem services.
- Django considers file manipulations that are NOT under MEDIA_ROOT
  to be suspicious, probably with good reason.

With this commit, ddrlocal and webui use settings.MEDIA_BASE
rather than storage.base_path().