Fix uniqueness constraint errors when importing github issues with multiple assignees

What does this MR do?

This MR makes it possible to do a bulk insert ignoring uniqueness constraints if specified.

This solves importing issues with multiple assignees if the issue import is retried, or the assignee is specified multiple times for some reason.

By using a database feature for this, we do not have to increase the number of queries performed.

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
  • Has been reviewed by Database
• Conform by the merge request performance guides
• Conform by the style guides
• Squashed related commits together

What are the relevant issue numbers?

To investigate further