Use joinPaths to build URL

What does this MR do?

As suggested in !40316 (comment 401874511) joinPaths does support building absolute URLs. So this MR converts the URLs introduced in !40316 (merged) from the strings to joinPaths constructions.

Screenshots

No visible changes.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done