fix: persist unit preferences in config directory instead of package directory

[motamman]

• Unit preferences config, custom definitions, custom categories, and custom presets were being
  read from and written to the npm package directory (node_modules/signalk-server/unitpreferences/), causing all customizations to be lost on upgrade
  • On global installs (e.g. sudo npm install -g), the server process couldn't write to the root-owned package directory at all, resulting in 500 errors when saving preferences.
  • Mutable files are now read from and written to ~/.signalk/unitpreferences/, with built-in presets and standard definitions still served from the package directory
  • On first run, the config directory is created and seeded with defaults from the package

@tkurki ... this one is entirely on me. Can you look over my shoulder?

[tkurki]

Draft by accident or on purpose?

[motamman]

Draft by accident or on purpose?

accident