- Represent the path to an Enterprise Edition license file using the
  environment variable `GITLAB_LICENSE_FILE`
- Document how to properly configure `GITLAB_LICENSE_FILE` environment
  variable in source and omnibus installations.
- Detect and automatically install the GitLab license during source
  installation if present in the `config` directory and named
  `Gitlab.gitlab-license`

Resolves: https://gitlab.com/gitlab-org/gitlab-ee/issues/10808Signed-off-by: Robert Marshall <rmarshall@gitlab.com>