Search for issues with multiple assignees

What does this MR do?

Implements filter for issues by multiple assignees (AND logic)

For now, it contains BE part only. FE part is here https://gitlab.com/gitlab-org/gitlab-ee/merge_requests/1494

Are there points in the code the reviewer needs to double check?

Why was this MR needed?

Screenshots (if relevant)

Does this MR meet the acceptance criteria?

• Changelog entry added, if necessary
• Documentation created/updated
• API support added
• Tests
  • Added for this feature/bug
  • All builds are passing
• Conform by the merge request performance guides
• Conform by the style guides
• Branch has no merge conflicts with master (if it does - rebase it please)
• Squashed related commits together

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ee/issues/1967