Escape ref and path for relative links

What does this MR do?

Escape ref and path in RelativeLinkFilter.

Why was this MR needed?

ref and path come in unescaped (see also discussion at !5878 (merged)) but Addressable::URI expects the components to be escaped. This lead to relative links breaking for directories which contain spaces or branch names containing %.

What are the relevant issue numbers?

fixes #21420 (closed)