Fix wrong links to GitLab when server is behind a proxy

[lmife]

Actually, the functionality was already there. Switched from using the GitLab Server URL from the Jenkins config to the public web URL provided by the GitLab server itself. Also fixed wrong LOGGER calls.

This fixes #380

Testing done

Used in test and staging environment. Checked all links inside the scan log, project page, branch page and build page. All links are using the correct URL, now.

### Submitter checklist
- [x] Make sure you are opening from a **topic/feature/bugfix branch** (right side) and not your main branch!
- [x] Ensure that the pull request title represents the desired changelog entry
- [x] Please describe what you did
- [x] 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

[jetersen]

LGTM