Add repository visibility field

[bradrydzewski]

I would like to add a new attribute to the repository called visibility. This will give us more control over which repositories are visible to users and under which circumstances. This idea is inspired by gitlab.

The visibility field will have the following values:

• public visible by anyone
• private visible by anyone with access
• internal visible by any authenticated user

The visibility field will have the following default value:

• public if repository is public
• private if repository is private

The use case for this is that some organizations are running github in private mode (meaning the repository is private) but don't want users to have to login to see the build results. This will replace the undocumented PUBLIC_MODE hack that we added a while back.

[bradrydzewski]

closed by #2054

[bradrydzewski]

note that at the moment this field can only be set via the command line utility (compiled from head or version 0.7+ if you are reading this in the future)

$ drone repo update octocat/hello-world --visibility=public
$ drone repo update octocat/hello-world --visibility=private
$ drone repo update octocat/hello-world --visibility=internal
$ drone repo info octocat/hello-world
  Owner: octocat
  Repo: hello-world
  Type: git
  Config: .drone.yml
  Visibility: internal
  Private: false
  Trusted: false
  Gated: false
  Remote: https://github.com/octocat/hello-world