add line expansion to side-by-side view of diffs in merge requests

Description

Sometimes (e.g. when editing text) the inline diff is hard to read, because one needs to see an entire section as is, without parseing which lines to read and which to skip. for this, the side-by-side view is super nice. However, one may need more context than visible in the diff (to read the previous paragraph, or the entire section, to review the order of paragraphs). This feature is available with the dots (see red marked screenshot) in the inline diff, but not in the side-by-side view.

Proposal

add the "dot-dot" expansion in the side-by-side view.