Make IS_GITLAB_EE consistent

Currently:
1. If `IS_GITLAB_EE=` is used, it fails to generate Webpack
   as it cannot be parsed as JSON,
2. If `IS_GITLAB_EE=1` is used, it fails on GitLab CE,
3. If `IS_GITLAB_EE=0` it does work correctly, and disables EE,
4. If `IS_GITLAB_EE=1` is used on GitLab EE it effectively required
   to run on EE changes.

This changes to:

Make `IS_GITLAB_EE=` to donate the intent to disable or enable EE
sources, instead of requiring it, and makes the check consitent
in behaviour.