Use stored user mapping for Jira importer

What does this MR do?

This MR takes mapping that was implemented in previous MRs and applies them on issue authors and assignees.

This is the final MR for mapping assignees & authors of issues in Jira importer (iteration I)

Related MRs

• Fetching users from Jira graphql endpoint (merged)
• Preparation of mapping caching (merged)
• User mapping form (FE)
• Storing submitted mapping in the cache
• (Automatic Jira->Gitlab users mapping (merging before this MR not needed)

WIP

• This can be merged once all related MRs are merged
• This should be delivered with documentation

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.

Related issue: #214810 (closed)