Migrate button to add previously merged commits

What does this MR do and why?

Migrates the button to add previously merged commits to an MR.

Before	After

How to review

First button

• Go to a MR that has at least one commit
• Remove .add-review-item-modal-trigger from https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/projects/commits/_commits.html.haml#L26 (otherwise the button gets replaced by JS)
• Refresh the page and scroll down to "Add previously merged commit"

Second button

• Add back the class you removed in the previous step and instead remove the same class from https://gitlab.com/gitlab-org/gitlab/-/blob/master/app/views/projects/commits/_commits.html.haml#L44
• Refresh the page
• Click on "Add previously merged commit" and add at least one commit in the modal
• You should now see the second button in the line with "x previously merged commits"

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.

Related to #363916 (closed)