minidlna: Add storage location dropdown

https://discuss.freedombox.org/t/minidlna-media-location-drop-down-menu-option/1443/1

• Default location: /var/lib/minidlna
• Extras locations:
  • /var/lib/transmission-daemon/downloads
  • /var/lib/deluged/Downloads
  • /var/lib/freedombox/shares/open_share
  • /var/lib/freedombox/shares/group_share

(Although may be overridden in the longer term by #1869.)