Prevents ID values automatically generated by headers in GitLab Flavored Markdown from colliding with IDs used elsewhere in the GitLab interface. This can cause confusion when, for instance, a selector looks for a merge request tab with id="pipelines" and there is a header with the same ID earlier in the DOM.
How this works:
All header IDs generated with GitLab Flavored Markdown are namespaced with id="user-content_foo"
All anchor links which point to these IDs continue to use the non-namespaced hash <a href="#foo">...</a>
When a page is loaded or when the hashchange event is triggered, javascript will automatically search for #user-content_foo if #foo cannot be found, and scroll to that position instead.
