JENKINS-50406 "Triggered Build" links broken with cloudbees-folder

[sophistifunk]

Fix the issue with downstream build URL generation, and add some snarky FIXMEs which I will address in a seperate PR

Description

See JENKINS-50406.

Submitter checklist

• Link to JIRA ticket in description, if appropriate.
• Change is code complete and matches issue description
• Appropriate unit or acceptance tests or explanation to why this change has no tests
• Reviewer's manual test instructions provided in PR description. See Reviewer's first task below.

Reviewer checklist

• Run the changes and verified the change matches the issue description
• Reviewed the code
• Verified that the appropriate tests have been written or valid explanation given

[sophistifunk]

I will address the new FIXMEs in a separate PR tomorrow, as it will need to be done carefully in order not to mess with Thor and co, and will take a few hours.

[NicuPascu]

LGTM, looking forward to having all url utils in one place