Merge branch 'fix-bulk-assign-labels-init' into 'master'

Fixes bulk-assign labels not working when navigating through pages

## What does this MR do?
This MR fixes an issue on bulk-assign labels not working when navigating though pages. The functionality was not working because events on checkboxes weren't working. This re-runs `Issuable.initChecks()` and makes sure we are not leaving zombie event handlers by turning them off first.

Also fixes tests to use `wait_for_ajax` since label filtering is now using `Turbolinks` for redirection.

## Does this MR meet the acceptance criteria?

- [ ] [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
  - [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)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4632