Don't create todos for old issue assignees

What does this MR do?

It creates Todos only for new assignees and not for the old ones.

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

Why was this MR needed?

When issue assignees were changed we created a new todo for all assignees including the ones that were already assigned to the issue.

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
• 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

What are the relevant issue numbers?

Closes #3642 (closed)