Refactor CI variable list code for usage with CI/CD settings page secret variables

What does this MR do?

EE MR, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4274

• Refactor CI variable list code for usage with CI/CD settings page secret variables, see https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110
  • We don't replace the CI/CD settings page secret variables in this MR, just the refactor
• Only user-facing change is we now hide/reveal the values on the pipeline schedule page

/<namespace>/<project>/pipeline_schedules/<iid>/edit

Before	After

Are there points in the code the reviewer needs to double check?

• This MR relies on https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16689 and https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/16699 so I rebased this on a branch that includes those commits but they shouldn't show up in the diff here.
• Previously setup_pipeline_variable_list.js handles this but it has been refactored into a general ci_variable_list.js which can now handle key, value, protected checkbox, and environment dropdown. Pipeline schedules doesn't utilize the protected checkbox or environment dropdown code but you can see them here, https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/4110
• We are keeping the EE-specific environment dropdown JS code in EE
  • It won't be used unless the HAML renders it(that only happens in EE)
  • It makes it easier to understand what is going on (no EE super class)
  • Conflict free
  • We also do this for the IDE code (in CE but has some EEU features)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• End-to-end tests pass (package-qa manual pipeline job)

What are the relevant issue numbers?