Remove adjacent // in relative urls

What does this MR do?

Removes hard-coded adjacent // in relative URLs.

This causes future URLs to carry the // in the URL path, and adds another / each time they are clicked.

Screenshots

https://gitlab.com/gitlab-org/gitlab/uploads/0f115f5181144cade1c85e9dcc1881fe/image.png

#214878 (moved)

slack (internal)