WIP: allow symfony 3
it seems we need to adjust the form handling to make symfony 3 happy.
we could try to use the approach sonata did: https://github.com/sonata-project/SonataAdminBundle/commit/3e7ad359411a5f4ae119d9dc3ef26b3e44716fd1#diff-baa44e28e0e16fee196ae10cb9a3fb46
or something like here https://github.com/sonata-project/SonataDoctrinePhpcrAdminBundle/pull/350
Also, I couldn't make it work with the @session service, I think the RequestStack should be passed instead, but not quite sure here
i think we should just bump the minimal version to symfony 2.8 so that we don't have to worry about supporting old symfony versions that reached end of life anyways.
i don't plan to work on this. @kix do you want to take this up?