link between frontend and backend

[wouterj]

Migrated from sonata-project/SonataDoctrinePhpcrAdminBundle#116. Original description:

• with [AdminExtension] Add link to page in frontend if possible routing-bundle#273 you can have a link from the edit view of a document to its
  frontend representation.
• Provide a link to the frontend in the list view too.
• provide a sonata link from the frontend in the create toolbar for the mainContent.

[dbu]

the create toolbar is not relevant anymore with create no longer maintained