Connect internal GitLab hosts on self-hosted

Self-hosted dashboards could not connect a GitLab group or Bitbucket project on an internal hostname, because the instance URL had to resolve to a public IP address, a protection that only makes sense for the cloud service.

• Fixed the GitLab and Bitbucket setup on self-hosted installs to accept instance URLs on internal DNS
• Kept HTTPS mandatory for the instance URL, since the access token travels over it