Refactor GitAccess and related code to remove build_download_code

The following discussion from gitlab-ce!11398 should be addressed:

• @DouweM started a discussion: (+5 comments)

  Do you understand why we have a separate build_download_code and download_code? :/

  /cc @ayufan

  ...

  What we want to accomplish is that GitAccess gives admin users access to all projects unless the access check is coming from a CI build, in which case GitAccess should only allow access to projects the admin has access to through normal non-admin means, like being a member, the project being public etc.

  We could accomplish this by telling GitAccess that the request is coming from a CI build, and not just checking can?(current_user, :download_code, project) in this case, but also verifying the reason for this access.