Add container registry support

Tasks:

• Merge docker/distribution authentication service: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/3787

• Implement Docker Registry API

• Show a list of docker images in GitLab

• Remove registry repository on project deletion

• Support project rename, move and namespace rename

• Use token when connecting the registry

• Allow to delete images from GitLab

• Support pushing from GitLab CI (gitlab-ci-token / $CI_BUILD_TOKEN)

• Support GitLab Runner pulling for public repositories

• Support GitLab Runner pulling for private repositories

• Add tests for Docker Registry API

• Add tests for a views

• Make texts nicer

• Implement a backup support

• Create administration documentation https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4141

• Create user documentation https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/4244