Add networking support (part 1)

[Tug]

This PR is required for task #202

First it fixes how gutenberg is loaded on boot: we need to initialize some data middleware before calling our App component. Just like gutenberg is doing for the web in the plugin (server-side) part: https://github.com/WordPress/gutenberg/blob/master/lib/client-assets.php#L205

TODO:

• Get an authentication token + a blog url param from the native app and use it to set up the apiFetch middleware. We can get some inspiration from calypso for this https://github.com/Automattic/wp-calypso/blob/master/client/gutenberg/editor/api-middleware/index.js#L199

[etoledom]

Tested with the getMedia( id ) call from Image block, and it effectively call apiFetch with the corresponding path.
As you mentioned, now we need to make that call work.

Thank you so much @Tug ! This one was for sure a tricky one! ✨

[Tug]

Thanks for reviewing @etoledom :)
Let's merge this and continue the work in another PR!