Extract EE specific files/lines for app/assets/javascripts/boards

What does this MR do?

The goal of this MR is to improve the quality of the code by moving EE specific lines out of app/assets/javascripts/boards. No behavior should change.

This requires some minor refactoring of the existing code to allow overriding behavior in EE.

This MR refactors the following files:

• app/assets/javascripts/boards/components/modal/footer.js
• app/assets/javascripts/boards/components/sidebar/remove_issue.js
• app/assets/javascripts/boards/services/board_service.js
• app/assets/javascripts/boards/models/issue.js
• app/assets/javascripts/boards/models/list.js

This MR does not include refactoring for:

• app/assets/javascripts/boards/index.js
• app/assets/javascripts/boards/stores/boards_store.js

These files included some very significant differences between CE and EE and should probably be handled in another MR.

Why was this MR needed?

#5787 (closed)

Does this MR meet the acceptance criteria?

• [ ] Changelog entry added, if necessary
• [ ] Documentation created/updated
• [ ] API support added
• [ ] Tests added for this feature/bug
• Conform by the code review guidelines
  • [ ] Has been reviewed by a UX Designer
  • Has been reviewed by a Frontend maintainer
  • [ ] Has been reviewed by a Backend maintainer
  • [ ] Has been reviewed by a Database specialist
• EE specific content should be in the top level /ee folder
• Conform by the merge request performance guides
• Conform by the style guides
• If you have multiple commits, please combine them into a few logically organized commits by squashing them
• Internationalization required/considered
• [ ] If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?

#5787 (closed)

Links

• gitlab-ce!19486