Attach blocks: The faster way

The API should allow developer to attach other type of content, not just block, which is too slow for dev time.

```yml
routes:
  atest_route/blocks:
    title: Hello Blocks
    access arguments: ['access content']
    content: >
      <h1>Main content here!</h1>
      Hello!
    blocks:
      seven:
        help:
          - 'system:powered-by'
          - ['user:online', {title: "Online users", weight: -100}]
          - delta: fancy-block
            subject: 'Fancy block'
            content: { content: 'Hey Andy!' }
            weight: 1000
```