Fix #1910: show a Toast message when blog creation is successful

Fix #1910 

Note: I don't want to show an error message [here](https://github.com/wordpress-mobile/WordPress-Android/blob/b08e49f1bfff7adae9e3fa3d22a2fa9afeec1d4f/WordPress/src/main/java/org/wordpress/android/ui/accounts/NewBlogFragment.java#L209), because (1) it-can't-happen™ and (2) it would means something like: "WordPress.com blog successfully created but local setup failed because our REST api returned something wrong". Other errors are reported by [this part](https://github.com/wordpress-mobile/WordPress-Android/blob/b08e49f1bfff7adae9e3fa3d22a2fa9afeec1d4f/WordPress/src/main/java/org/wordpress/android/ui/accounts/NewBlogFragment.java#L216-L222)