Merge branch 'rs-bulk-issues-update-spec' into 'master'

Refactor Issues::BulkUpdateService spec

- Create fewer Issue objects; 2 is as good as 5 for these cases. This
  gives us a nice little speed improvement.
- Don't `describe` Symbols.
- Simplify object creation.
- Lessen "mystery guest" antipattern

See merge request !5380