Fix unwanted label unassignment

## What does this MR do?
- When updating the milestone
  - [x] Do not remove labels when assigning a milestone
  - [x] Do not remove labels when unassigning a milestone
  - [x] Do not remove labels when assigning a milestone and adding another label

- When toggling selected issues labels should be kept
  - [x] Select an issue with an assigned label -> pick another label from dropdown-> unselect the issue -> select the issue again -> submit the form: Existing label should not be removed.

## Are there points in the code the reviewer needs to double check?
Labels should not be added or removed to issues when doing bulk actions unless we explicitly select a label from the dropdown

## 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
  - [x] Added for this feature/bug
  - [x] All builds are passing
- [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)
- [ ] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4863