Feature request: post pin-to-top

[msg7086]

As a blogger, I'd like to pin a post to the top of my blog so it appears before all other posts.

Todo:

• https://github.com/pagekit/extension-blog/blob/master/src/Model/Post.php
  Add pinned bollean field.

• https://github.com/pagekit/extension-blog/blob/master/src/Controller/SiteController.php#L42
  Order by pinned first.

• Admin edit page: Add pinned checkbox.

• Some other minor places.