[1.1] Improve Block admin

[wouterj]

Instead of showing one admin per block type, the admin should have just one block admin. This would start with a type field, which dynamically changes the form fields and created document based on the selected type.

We should also refactor the container admin block to have a nice drag 'n drop view. E.g. like this:

(source: https://buytaert.net/examples-of-how-to-make-drupal-outside-in)

[ElectricMaxxx]

Btw: i would implement a sonata decoupled component (mostly javascript frontend) to use here and every where else. maybe implemented as an form type here.