management commands

[benzkji]

• eliminate duplicates (with config, update filer fields, hint about related_name='+', allow dry run!)
• delete all thumbnails (dry run?)
• delete orphaned files (dry run!)
• delete_unused_files (....dry run)
• apply get_filename (to replace /x8/ad67/lnasdkva879sd7tva68sdva9sd6v87as6df/myfile.pdf with for example /myfile.pdf or /sad8d7yc/myfile.pdf)
• remove empty directories from filesystem/storage (remove unused directories (on filesystem/storage) #12)