Map jira issue assignee and author

What does this MR do?

It adds actual user mapping for issue author and assignee during the Jira import.

It takes the email we get from Jira and searches for the GitLab user based on that email address. The query should always find max 1 user.

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• [-] Documentation (if required)
• Code review guidelines
• [-] Merge request performance guidelines
• Style guides
• [-] Database guides
• [-] Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.

Issue: #215894 (closed)