refactor: Use functional components instead of class components

[pahund]

The AdvertisingProvider is really messy, best would probably to rewrite it from scratch as functional component. This should also fix the warning message we currently have when running Jest tests:

 PASS  src/components/AdvertisingProvider.test.js (7.431 s)
  ● Console

    console.error
      Warning: Can't perform a React state update on an unmounted component. This is a no-op, but it indicates a memory leak in your application. To fix, cancel all subscriptions and asynchronous tasks in the componentWillUnmount method.
          at AdvertisingProvider (/Users/pahund/git/react-advertising/src/components/AdvertisingProvider.js:9:22)

      38 |       await this.advertising.setup();
      39 |       // eslint-disable-next-line react/no-did-update-set-state
    > 40 |       this.setState({
         |            ^
      41 |         activate: this.advertising.activate.bind(this.advertising),
      42 |         config: this.advertising.config,
      43 |       });