Backport 556582-link-to-project-not-working-when-gitlab-hosted-in-subpath-after-upgrade-to-18-2

What does this MR do and why?

In !196877 (merged) we were trying to fix some issues that customers were having around absoulte URLs by switching to relative URLs. This unfortunately caused a new issue when using the relative_url_root setting.

We fixed the issue in !200577 (merged). This MR cherry-picks the commit from that MR to backport that fix to 18.2

The merge commit relied on code from two other commits so I had to include these:

• 6453a99e
• ba2d2ecd

Both of these are fixing bugs behind feature flags (your_work_groups_vue and admin_groups_vue) which are both disabled by default in %18.2

Note to maintainer: this is not an ideal fix but the best option for a quick fix. I have two issues open to improve this in the future:

• Improve developer tooling around URLs in GitLab (#560240)
• Pass admin paths from backend to frontend in Vu... (#560248)

Screenshots

Notice "Before" is missing the /gitlab prefix

Before	After

How to set up and validate locally

1. Add relative_url_root: '/gitlab' to your gdk.yml file in the gitlab-development-kit directory
2. Run gdk reconfigure
3. Run gdk restart
4. Go to /gitlab/dashboard/projects. URLs should work

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• This MR is backporting a bug fix, documentation update, or spec fix, previously merged in the default branch.
• The MR that fixed the bug on the default branch has been deployed to GitLab.com (not applicable for documentation or spec changes).
• The MR title is descriptive (e.g. "Backport of 'title of default branch MR'"). This is important, since the title will be copied to the patch blog post.
• Required labels have been applied to this merge request
  • severity label and bug subtype labels (if applicable)
  • If this MR fixes a bug that affects customers, the customer label has been applied.
• Set the milestone of the merge request to match the target backport branch version.
• This MR has been approved by a maintainer (only one approval is required).
• Ensure the e2e:test-on-omnibus-ee job has either succeeded or been approved by a Software Engineer in Test.

Note to the merge request author and maintainer

If you have questions about the patch release process, please:

• Refer to the patch release runbook for engineers and maintainers for guidance.
• Ask questions on the #releases Slack channel (internal only).
• Once the backport has been merged, the commit changes will be automatically deployed to a release environment that can be used for manual validation. See after merging runbook for details.