Add option to limit the number of assignees of a merge request

Like #343362, the number of assignees is also uncapped. Since this fix should be fairly trivial if the work has already been done for reviewers, we could add this safeguard as well.

As mentioned in the related issue:

Having too many assignees could lead to potential performance problems. In addition, the user experience will degrade as we add more and more assignees.

This issue is to add a limit to the number of assignees that are allowed on a merge request.