Introduce horizontal rules in the MR widget

This issue is part of https://gitlab.com/gitlab-org/gitlab-ce/issues/33095

Design/Proposal

• Newer designs already introduce horizontal rules <hr>

*This should be done sensibly (grouping similar points together) ... which means that subitems without the treelines as of https://gitlab.com/gitlab-org/gitlab-ce/issues/33982 belong to 1 section along with their main point! See the mockup:

cc: @victorwu (making the https://gitlab.com/gitlab-org/gitlab-ce/issues/33095 more readable)