Export code as ES6 modules

What does this MR do?

Export code as ES6 modules:

1. app/assets/javascripts/milestone.js
2. app/assets/javascripts/new_branch_form.js
3. app/assets/javascripts/new_commit_form.js
4. app/assets/javascripts/subscription_select.js

Does this MR meet the acceptance criteria?

• Review
  • Has been reviewed by Frontend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

#38869 (moved)