Implemented EZP-20238 - Upgrade to Symfony 2.2

See also https://github.com/ezsystems/ezpublish5/pull/31

## Tasks

- [x] **Semantic configuration**: Use `normalizeKeys( false )` in config tree to avoid key normalization whenever needed (see https://github.com/symfony/symfony/pull/6367)
- [x] **HttpCache**: `LocationAwareStore` should now be based on `Symfony\Component\HttpKernel\HttpCache\Store` (see https://github.com/symfony/symfony/pull/6016)
- [x] **Twig**: Default Twig loader is now `Twig_Loader_Chain`. We can now remove the twig loader filesystem override, and keep `twig.loader.string` (See https://github.com/symfony/symfony/commit/f48b22a44ea171404471f4371d5f0242705c5f80)
- [x] Ensure [Symfony 2.2 upgrade instructions](https://github.com/symfony/symfony/blob/master/UPGRADE-2.2.md) are followed
- [x] Replace deprecated Symfony API calls by new ones (see https://github.com/symfony/symfony/blob/master/UPGRADE-3.0.md)
- [x] Fix potential regressions !