Refactor jQuery dropdown implementation to GitLab UI GlDropdown in init_changes_dropdown.js

The following code utilizes our old deprecated jQuery-based dropdown implementation and should be replaced with a new implementation based upon GitLab UI's GlDropdown component:

• app/assets/javascripts/init_changes_dropdown.js
• app/views/projects/diffs/_stats.html.haml

This dropdown is visible on diff pages within merge requests, commits, or revision comparison views:

Part of &4284