Use router instead of internal component state

[psrpinto]

Previously, the BlogPostFlow component was handling transitions between screens internally, instead of relying on the URL to drive navigation.

This PR fixes that by removing this component in favor of two others, which are mapped to two different URLs:

• NewBlogPost: /session/${ sessionId }/flow/blog-post/new
• EditBlogPost: /session/${ sessionId }/flow/blog-post/${ postId }

This makes the app behave in more standard "CRUD" way.

Additionally, some other small changes were made:

• Remove duplicated code for retrieving information about the current source site
• Make it possible to navigate from the dev tools: e.g. window.trywp.navigateTo( '/start/home' )
• Add a Toolbar component for rendering the buttons of each screen (see screen capture below), so that the "next" button is always on the top right.

Screen captures

[ashfame]

LGTM!