Sandbox: Update to symfony 3.0.x

Problem/Motivation

As discussed in #2395443: [policy, no patch] Follow symfony 2.7 or 3.0., this is the question we maybe switch over to symfony 3.0.x at some point.
In order to be able to do that, you need a way to ensure that nothing is broken, when we update.

Proposed resolution

Continuously try to update to symfony 3.0.x to see what BC apis we should try to replace with new ones,
where symfony 3.0.x maybe introduces new bugs etc.

Blocking issues:

• https://github.com/symfony/symfony/issues/13140
• https://github.com/symfony/symfony/issues/13139
• https://github.com/symfony-cmf/Routing/issues/123

Remaining tasks

User interface changes

API changes