Rewrite of Starter template

The Custom Application Starter template has been rewritten from scratch, to provide a better experience to new developers working on Custom Applications.

Below an overview of the most important changes:

• The template now has more built-in dev tools, like ESLint and Prettier.
• The template README has been cleaned up a bit.
• The template uses the new React JSX transform.
• The template uses the new menu links in application config.
• The template has a nicer landing/welcome page with useful links to get started.
• The template has a channels page, that fetches channels and renders them in a table. This is a good showcase of some of our best practices, for example to use hooks, to use GraphQL, to use notifications, to render data in a table (with pagination), etc.
• The template has React-Testing-Library tests, including mocking GraphQL and using test data.

Furthermore, the @commercetools-frontend/create-mc-app package now automatically derives the entryPointUriPath from the folder name when installing the template. The value is then applied to the application config and other places in the source code.