Potential approach for changing ezp-next to use Composer and Symfony Con...

...tainer

!! DO NOT MERGE: this currently breaks all tests !!

Not sure if this is the way it should be done, but here is an initial commit
for changing bootstrap.php and container.php to stop using custom autoload,
configuration, container and ini settings, syncing with the approach in eZ Publish 5.

This approach makes sure bootstrap and container works both in the context of API
(ezp-next) as standalone, and also within ezpublish5 context.
Meaning there is no longer a need to install vendors in both eZ Publish 5 and API when
in ezpublish5 context (builds).

This is purely for starting an discussion on how we can expose container to command line
scripts and integration/unit tests (they need to be able to get Repository object).

If this approach is taken, then the following remains:
- cleanup /  change tests to not check for legacyKernel env variable but instead get it via injection/ via service container
- default settings might need to be moved from ezpublish5 to API (ezp-next)
- Service tests that manipulates the container needs to be modified
- (..)