This is a follow-up of request #14384

Now that even legacy files are concatenated with Webpack, we should harmonize plugins. /plugins/<plugin-name> should be the "entrypoint" for all plugins. It should contain:

• package.json & package-lock.json. As long as it makes sense. Some dependencies still need to be scoped per-application, for example in agiledashboard: kanban and planning-v2 should not share dependencies.
• jest.config.js
• tsconfig.json
• webpack.(common|dev|prod).js

This setup will make it easier for everyone to know where they should run "npm" commands. It also lets us unify themes and scripts builds by having only one command to build / watch both.