Rely on mpv to know where to store application data & cache

[nbraud]

• Put the uid file and database under ~~state and ~~cache respectively
  This is the most platform-independent way, as we can trust mpv to do the right thing everywhere.
  Closes Use XDG_DATA_HOME and XDG_CACHE_HOME if defined for UID and DB #17
• Automatically migrate those files from the previous location
  This makes the change completely transparent for users
• Use mp.find_config_file to find sponsorblock.py
  It otherwise breaks for users who have a global scripts/ dir, as mentioned in enable loading from global scripts dir #66

[nbraud]

@po5 I hope this addresses the concerns you expressed in #17, about working on all platforms, and relying on mpv as the source of truth