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 through 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 entry added
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the style guides
• Branch has no merge conflicts with master (if you do - rebase it please)
• Squashed related commits together