Statamic 5 Support

[duncanmcclean]

This pull request adds Statamic 5 support to the v2 Migrator package.

To Do

• [x] Get the tests passing
• [x] Migrate statamic.system.multisite to true when Site::hasMultiple()
  • We used to infer this, but now have explicit boolean
  • The SettingsMigrator should handle this