Fix link to GitLab when server ends with a slash

[jmini]

Fixes #329

The GitLabHelper now handles the case where the server ends with a / when computing links.

Testing done

Tested in a local instance

Submitter checklist

• Make sure you are opening from a topic/feature/bugfix branch (right side) and not your main branch!
• Ensure that the pull request title represents the desired changelog entry
• Please describe what you did
• Link to relevant issues in GitHub or Jira
• Link to relevant pull requests, esp. upstream and downstream changes
• Ensure you have provided tests - that demonstrates feature works or fixes the issue

[jmini]

@jetersen can you have a look?