Add gitlab omniauth template

What does this merge request do and why?

This MR will add a template to set up gitlab.com omniauth settings via gdk.yml

How to set up and validate locally

1. Go to https://gitlab.com/-/profile/applications and create an new application with scope read_user set the callback URL to http://YOURGDKHOST:3000/users/auth/gitlab/callback
2. Note the app ID and app secret
3. Edit gdk.yml to contain:

omniauth:
  enabled: true
  gitlab:
    app_id: 'NOTED APP_ID'
    enabled: true
    app_secret: 'NOTED APP_SECRET'

After gdk reconfigure and gdk restart the login screen should show the GitLab.com login option:

Impacted categories

The following categories relate to this merge request:

• gdk-reliability - e.g. When a GDK action fails to complete.
• gdk-usability - e.g. Improvements or suggestions around how the GDK functions.
• gdk-performance - e.g. When a GDK action is slow or times out.

Merge request checklist

• This change is backward compatible. If not, please include steps to communicate to our users.
• Tests added for new functionality. If not, please raise an issue to follow-up.
• Documentation added/updated, if needed.
• Announcement added, if change is notable.
• gdk doctor test added, if needed.
• Add the ~highlight label if this MR should be included in the CHANGELOG.md.