Resolve "Webhook Image URLs Rewritten Incorrectly in Issues"

What does this MR do?

Fixes a bug in https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/18109: uploads aren't actually absolute on the server, but rewritten to be relative to the project.

For instance, ![image](/uploads/e58fc9164738773b8cbeef37ea173223/image.png) renders as the below. (It's supposed to be a white box, it's not broken!)

And the full URL for this image is: https://gitlab.com/gitlab-org/gitlab-ce/uploads/e58fc9164738773b8cbeef37ea173223/image.png

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug

Closes #52650 (closed).