Create a new job to check that EE-specific files are added to the top-level ee/ folder

We're in the process of moving all the EE-specific files to the ee/ top-level folder (&27 (closed)), but we regularly see new EE-specific files added to the CE codebase, so we're fighting a battle that will never ends.

The idea is to introduce a new job that would check that EE-specific files are not added to folders other than the ee/ top-level folder.

Something like that:

1. git fetch https://gitlab.com/gitlab-org/gitlab-ce.git master
2. git diff FETCH_HEAD..HEAD --name-status --diff-filter=A -- ./ ':!ee' | cut -f2

This last command should have an empty output.

Note: We will need to have a whitelist until we have moved every EE-specific files to ee/.