Add a kernel again

[bytehead]

Because it's gone for prefer-dist installations: contao/contao#762.

It also reverts #6.

Should be released as a new feature release 0.13.0 while Contao 4.9.0 is released.

To-Do:

• Update Composer constraint to "contao/core-bundle": "^4.9"

[leofeyer]

I thought we did not want to merge this and keep using the external kernel so we do not have to maintain the config.yml and security.yml files in several different places?

[bytehead]

The master build failed because of contao/contao#762. But I can revert it if you want. The config.yml and security.yml don't change that much IMO.

[leofeyer]

I think the correct fix for that would be to install from source instead from dist:

    "config": {
        "preferred-install": {
            "contao/*": "source",
            "*": "dist"
        }
    },

[bytehead]

Revert PR: #13