Use flexible types for Article

Instead of hardcoding whether an article is a page or a post, just use the type in the current URL, so we can make more types for article work just by adding new urls
Add the 'block' type for Article, together with Blade helper

The block type is something you can use in templates to include the content from an dynamic block in your code, with:
  @block('block-slug')
Don't require an image to create a new article