Add container registry support

Tasks:

- [x] Merge docker/distribution authentication service: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3787
- [x] Implement Docker Registry API
- [x] Show a list of docker images in GitLab
- [x] Remove registry repository on project deletion
- [x] Support project rename, move and namespace rename
- [x] Use token when connecting the registry
- [x] Allow to delete images from GitLab
- [x] Support pushing from GitLab CI (gitlab-ci-token / $CI_BUILD_TOKEN)
- [x] Support GitLab Runner pulling for public repositories
- [ ] Support GitLab Runner pulling for private repositories
- [x] Add tests for Docker Registry API
- [x] Add tests for a views
- [x] Make texts nicer
- [x] Implement a backup support

- [ ] Create administration documentation https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4141
- [ ] Create user documentation

See merge request !4040