Refactor mapping users for jira import

What does this MR do?

It extracts code responsible for mapping jira users against gitlab users to a separate class.

We will reuse this code when importing all users at once in the beginning of the Jira importer.

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.