Extract EE-specific code to ee/ and spec/ee/ directories

We already have a meta issue for moving existing files at: https://gitlab.com/gitlab-org/gitlab-ee/issues/3107

However, some would require renaming strings and even the way how we resolute modules, files, and so on. They might not be trivial to move. At this point, I think we should focus on "extract" EE specific code first instead. Because those renaming could help code organization, but can't help for reducing conflicts.

The true conflicts offenders are those who directly modifying the code to extend EE features. We should fix them first. For example, https://gitlab.com/gitlab-org/gitlab-ee/issues/3377 is quite different from CE and EE, making it very hard to resolve conflicts, understanding what's the difference between CE and EE, and so on.

The best strategy was described in https://docs.gitlab.com/ee/development/ee_features.html

However, we don't have the framework for API yet, so we still just modify the API code in order to extend EE. I think this could be the top priority right now.

Edited by Rémy Coutable