added 245px max height and overflow scroll-y to .grouped-pipeline-dropdown

## What does this MR do?
It restricts the sub builds popup in the pipeline graph and makes it scrollable

## Are there points in the code the reviewer needs to double check?
none that I know of

## Why was this MR needed?
To restrict crazy CI configurations from messing with the viewport scrolling.

## Screenshots (if relevant)
![2016-10-05_16.29.35](/uploads/0ce1d8e4d46ec45e181919dd968a80cf/2016-10-05_16.29.35.gif)

## Does this MR meet the acceptance criteria?

- [x] [CHANGELOG](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CHANGELOG) entry added
- [ ] [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 you do - 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?

Closes #22295

See merge request !6696