Fix compatibility with Internet Explorer 11 for merge requests

What does this MR do?

This merge request restores the compatibility with Internet Explorer 11.

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

No.

Why was this MR needed?

Commit ca3c0c6c introduced an incompatibility with Internet Explorer 11. On all merge requests in all projects the 'Changes' tab does not display the changes in IE11 but instead fails with 'Something went wrong on our end'. The reason ist, that this code snipped produces different results on Firefox and Internet Explorer 11:

var element = document.createElement('a');
element.href = '/some/absolute/url';
alert(element.pathname);

With Internet Explorer 11 the alert will print a relative path, whereas with Firefox the alert will print an absolute path. For GitLab this meant that a wrong AJAX URL was composed which resulted in a 404.

Screenshots (if relevant)

None.

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 merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

#23977 (closed) #24380 (closed)