Move node rendering to Twig.

Creates a namespace called Legacy to contain legacy classes used by the
application, which are now auto loaded by Composer.

Rendering a text or image node is now on a very rude template, but it
servers the purpose of separating PHP code and HTML markup.

Wilber class was created to hold some helper methods to aid refactoring
the application twards a more decoupled design, currently it only has a
factory to Twig.