Issue export CSV includes "Weight" and "Locked"

What

Adds Weight and Locked to CSV export of issues

Why

Weight was omitted from https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1126, probably because that MR started as a CE MR. Similarly Locked could be included.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• Tests added for this feature/bug
• Review
  • Has been reviewed by Backend

Related

• https://gitlab.com/gitlab-org/gitlab-ce/issues/5747#note_67493864

Closes https://gitlab.com/gitlab-org/gitlab-ee/issues/2172 and https://gitlab.com/gitlab-org/gitlab-ee/issues/2287