FE: Add variables to pipelines schedules

What does this MR do?

FE for Add variables to pipelines schedules

BE: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12341

Integration (FE + BE): https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/12372

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

To view, go to a project -> Pipelines -> Schedules -> New schedule or edit an existing schedule

• Do I need to add the i18n/translation strings somewhere?

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
  • https://docs.gitlab.com/ce/user/project/pipelines/schedules.html (http://192.168.1.135:3000/help/user/project/pipelines/schedules.html#making-use-of-scheduled-pipeline-variables)
  • https://docs.gitlab.com/ce/ci/variables/README.html (http://192.168.1.135:3000/help/ci/variables/README.html)
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?