Add bulk update support for merge requests list

## What does this MR do?

Adds new feature: ability to update multiple merge requests at once from index page (same like we have for issues already)

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

I don't think so

## Why was this MR needed?

* consistency with issues
* save user time when you want set same label/milestone to 20 merge requests 

## Screenshots (if relevant)

![Screenshot_2016-09-08_11.48.49](/uploads/fe50b2472330e211eb8d6d6dafb15667/Screenshot_2016-09-08_11.48.49.png)![Screenshot_2016-09-08_11.48.46](/uploads/4833c3705120ec028f7a62a4e128998d/Screenshot_2016-09-08_11.48.46.png)

## Does this MR meet the acceptance criteria?

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

## What are the relevant issue numbers?

Fixes https://gitlab.com/gitlab-org/gitlab-ce/issues/20087

See merge request !6226