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

[nicoonoclaste]

• 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 #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 #66

[nicoonoclaste]

@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