Migrate diff buttons to pajamas component

What does this MR do and why?

Migrate buttons in app/helpers/diff_helper.rb to pajamas component.

It's a part of Pajamas adoption project

Changed according to migration guide available in next epic: &8153

MR acceptance checklist

Please evaluate this MR against the MR acceptance checklist. It helps you analyze changes to reduce risks in quality, performance, reliability, security, and maintainability.

Screenshots or screen recordings

Before	After

How to set up and validate locally

Case 1

1. Open some project.
2. Goto Code -> Commits from the side menu.
3. Click on commit to view it.
4. Check diff buttons on commit page(see screenshots).

Case 2

1. Commit some changes to new branch.
2. Create new MR.
3. Use diff buttons in Changes tab on New MR page.

Covered by:

spec/features/projects/commits/multi_view_diff_spec.rb

spec/features/merge_request/user_selects_branches_for_new_mr_spec.rb

Related to #417798 (closed)