Fix leading slash in redirects and add cop

What does this MR do?

Removes leading slashes in redirects that would cause 404 when gitlab is mounted under a subdirectory. Also adds a rubocop cop to avoid any future offences.

What are the relevant issue numbers?

Closes #50645 (closed) and #37423 (closed)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Conforms to the code review guidelines
• Conforms to the merge request performance guidelines
• Conforms to the style guides
• Conforms to the database guides

~Manage frontend regression regression:11.3