Add "engineering" UI for Pipeline Schedule (!10533) · Merge Requests · GitLab.org / GitLab Community Edition

Add "engineering" UI for Pipeline Schedule

What does this MR do?

We merged this awesome MR, but it was only backend change. This MR brings very simple engineering UI.

It does expose scheduled triggers only when editing page. It is hidden, because we give us a right to break that in 9.2 when we start working on better UI.

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

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

Changelog entry added, if necessary
Documentation created/updated
API support added
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?

## What does this MR do?
We merged this awesome [MR](https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/10133), but it was only ~backend change. This MR brings very simple engineering UI.

It does expose scheduled triggers only when editing page. It is hidden, because we give us a right to break that in %9.2 when we start working on better UI.

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

## Why was this MR needed?

## Screenshots (if relevant)
![Screen_Shot_2017-04-07_at_14.55.33](/uploads/2f1240ae14bd25d31049dd123f709914/Screen_Shot_2017-04-07_at_14.55.33.png)
![Screen_Shot_2017-04-07_at_14.55.12](/uploads/080af9deb83c8d27a390502feab7a6cc/Screen_Shot_2017-04-07_at_14.55.12.png)

## Does this MR meet the acceptance criteria?

- [ ] [Changelog entry](https://docs.gitlab.com/ce/development/changelog.html) added, if necessary
- [ ] [Documentation created/updated](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/doc/development/doc_styleguide.md)
- [ ] API support added
- Tests
  - [ ] Added for this feature/bug
  - [ ] All builds are passing
- [ ] Conform by the [merge request performance guides](http://docs.gitlab.com/ce/development/merge_request_performance_guidelines.html)
- [ ] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [ ] Branch has no merge conflicts with `master` (if it does - rebase it please)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

## What are the relevant issue numbers?