[Design] Lower the priority of the assignee in a merge request

Proposal

Make assignee less prominent when dealing with merge requests in GitLab.

This is an alternative approach to dealing with Remove Assignees from merge requests and replac... (#365969)

Assignees do have a purpose of conveying responsibility of a merge request in addition to the original author #365969 (comment 1794492070). So it is worthwhile to keep that feature there.

From the original issue that introduced this capability Multiple assignees for merge requests (#2004 - closed)

It is quite common for multiple people to work on the same merge request together, for example a backend engineer and frontend engineer. In the same way that it is helpful to have multiple assignees for an issue, it would be very helpful for multiple assignees fro merge requests.

Additionally, when reviewing a large merge request, it is not uncommon for multiple people to be reviewing the merge request at the same time. Assigning oneself is helpful for indicating which people are reviewing the merge request.

However the assignee field is not often used the way we have assumed #520163 (comment 2429063473). And the usecase for assigning someone as an assignee for indicating a review has been replaced by "Reviewers".

Therefore it shouldn't be the top thing in the sidebar and lists should reflect the concept of "my merge requests" rather than my assigned MRs.

We should update the GitLab Code Review Guidelines as well to reflect this

The author or directly responsible individual (DRI) stays assigned to the merge request as the assignee throughout the code review lifecycle.

Merge request authors and DRIs stay as Assignees.

Design

The boring solution would be to move assignees below "Reviewers"

A bigger change would be to highlight the author in the sidebar and have the ability to add additional assignees but I am unsure of what that label would be as "DRI" (directly responsible individual) or "Owners" are not the ideal labels.