Allow bulk-assign labels to issues

- [x] Allow bulk-assignment labels.
- [x] Show indeterminate state for labels present on selection.
- [x] Remove existing labels from selected items if label gets unmarked.
- [x] Fix conflicting tests.
- [x] Write tests.

Closes #13499 and #15489

See merge request !3917

Instead of passing `remove_label_ids`, just pass an empty array for
`label_ids` (and don't pass `add_label_ids` or `remove_label_ids`).

Triggering keyup will re-render the dropdown which is not needed when option persistWhenHide is true

This was re-rendering the dropdown unnecessarily

We need to update the results right away when we do bulk assignment.

1. Don't use instance variables, use `let` instead.
2. Add descriptions for all specs.
3. Share variables where possible.
4. Give labels more vivid names than 1, 2, and 3.
5. Remove deprecation warnings by passing issue IDs as '1,2,3' instead
   of an array, as that's how they're passed by the front-end. (The
   deprecation warning is for passing a nested array, which is what
   happens if an actual array is passed, as:
     `[1, 2, 3].split(',') == [[1, 2, 3]]`