Update Doctrine `AppBundle:Post` syntax to `Post::class`

[weaverryan]

I believe this is now the preferred syntax and writing it is "easier" a bit more universally (PhpStorm with the Symfony plugin will autocomplete both, but a normal IDE will only autocomplete the second).

This should be updated everywhere in the 3.1 branch (first, currently-maintained version with a high even PHP version to support this).

[xabbuh]

A quick note: Symfony 3.1 is not maintained anymore. ;) And I would by the way even make the change in the 2.7 branch as we already switched to the usage of FQCNs with the class constant in other places as well (to refer to form types for example).

This seems to be fixed in #7907.