Tidy up BulkUpdateService specs

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]]`