specify GraphicsMagick cmds for sorl
shorten MEDIA_ROOT; separate sorl cache and DDR base

Previously sorl cache and DDR collections were located in the same
directory.  However, sorl has problems if cache points to nonexistent
files, which would happen if USB HDD is changed.  This change puts
sorl cache dir and DDR base dir at the same level.  Sorl cache will
now be stored in VM instead of in USB HDD.

- Shorten paths: MEDIA_ROOT /var/www/media, STATIC_ROOT /var/www/static
- sorl-thumbnail cache: /var/www/media/cache.
- Symlink to DDR: /var/www/media/base.