[WIP] Start new SE

| Q             | A
| ------------- | ---
| Fixed tickets | https://github.com/symfony-cmf/symfony-cmf-standard/issues/31, https://github.com/symfony-cmf/symfony-cmf-standard/issues/42
| License       | MIT
| Doc PR        | todo (mainly quick tour)

This is the PR creating the new CMF SE. I've based it on the 2.3 branch of the core SE, because 2.3 is the LTS version. We may need to release a version including the 2.4?

Reducing
--------

That's the keyword of this PR. It tries to reduce the old CMF SE. What I've removed:

- Multi-language
- Media
- Imagine

I've also tried to keep the code as easy to understand as possible.

Todo
----

- [ ] Add more pages (3 pages max. imo)
- [ ] Add some front-end styling (using old SE style or core style?)
- [ ] Add tips about LiipDoctrineCacheBundle
- [ ] CreatePHP integration
- [ ] Add MenuBundle integration
- [ ] Update when https://github.com/symfony-cmf/RoutingBundle/pull/210 is merged