refs https://github.com/TryGhost/DevOps/issues/83
- this will now continue use the dev server assets if we tell it to,
or copy the dependency package files to the built folder otherwise
- removes `editor` from config API because it's no longer needed
- removes dependency on `editor.url` in tests, as this no longer exists
- edits dev script to pass dev server URL as env var
- adds `@tryghost/koenig-lexical` dependency to Admin
