A proper plugin example for reaction-admin

[zaibacu]

With:

• client side
• server side
• i18n

And no, current plugins aren't good examples. Couldn't find any example with i18n. Tried doing:

export default async function register(app) {
  await app.registerPlugin({
    <...>,
    i18n,
  });
}

But was unsuccessful.

[ghost]

Background Jobs are a potential use case as there's currently a functional library and documentation but not an API.