Use 'preload' in export to CSV

What does this MR do?

It assures that associated resources are fetched in a separate query instead of single query.

Why was this MR needed?

If .includes is used then Active Record uses JOIN to fetch associated resources under some circumstances (when also .references is used) this causes problem in combination with GROUP BY statement which is used by label filter and SQL query fails on:

ActiveRecord::StatementInvalid: PG::GroupingError: ERROR: column "users.id" must appear in the GROUP BY clause or be used in an aggregate function LINE 1: ..._r25, "issues"."service_desk_reply_to" AS t0_r26, "users"."i...

Use of .preload assures that Active Record always does a separate query to fetch associated resources.

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests added for this feature/bug
• Review
  • Has been reviewed by UX
  • Has been reviewed by Frontend
  • Has been reviewed by Backend
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together
• Internationalization required/considered
• If paid feature, have we considered GitLab.com plan and how it works for groups and is there a design for promoting it to users who aren't on the correct plan

What are the relevant issue numbers?

#4376 (closed)