Add Maven support to our Artifact Repository (MVC)

We want to add Maven support to our Artifact repository, to make it easier to work with for Java based projects/developers.

Suggested MVC:

• HTTP(S) based on GitLab's config (no WebDAV, etc.)
• Basic authentication
  • Use PAT token or CI Job Token, since our API supports that today
  • Should be able to simply use custom headers to pass authentication like we do today for API authentication
• Storage
  • Local and Object Storage
• Single repository with the name maven, no support for SNAPSHOT or specific plugin repos for now
• Utilize new Repositories API: https://gitlab.com/gitlab-org/gitlab-ee/issues/5838
  • Protocol seems to be simple HTTP(S) access with specific layouts, using GET/PUT's.

TODO:

• Backend support https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6607
• Workhorse support gitlab-workhorse!283 (merged)
• UI support https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6781
• Enable/disable packages per project https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/6977
• Omnibus support https://gitlab.com/gitlab-org/gitlab-ee/issues/7253
• Add support for the CI_JOB_TOKEN https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/7249

Further improvements

See https://gitlab.com/gitlab-org/gitlab-ee/issues/7678