Added support for dataporten

What does this MR do?

Added OAuth service Dataporten to GitLab.

Are there points in the code the reviewer needs to double check?

There have been commented additions to gitlab.yml.example. omniauth-dataporten has been added to the Gemfile and Gemfile.lock, with the dependencies required for it.

Why was this MR needed?

To make it easier to integrate Dataporten with GitLab

Does this MR meet the acceptance criteria?

• CHANGELOG entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together