Source files needed only by `python manage.py load_data`.
NOT required to run the web app — data/db.sqlite3 already has everything.

Expected layout:
  data/sources/bigdata_ld2_database.sqlite3   <- ld2 places/cities database
  data/sources/ld3_database.sqlite3           <- ld3 PageRank database
  data/sources/images/                        <- ld4 images root directory

You can also override paths via environment variables instead of copying files:
  export LD2_DB_PATH=/path/to/bigdata_ld2_database.sqlite3
  export LD3_DB_PATH=/path/to/ld3_database.sqlite3
  export LD4_IMAGES_ROOT=/path/to/images/