Add users mapping to Jira import start mutation and store it in Redis

What does this MR do?

It adds usersMapping argument to jiraImportStart mutation and then stores this mapping in the Redis.

Example

mutation {
  jiraImportStart(input: {jiraProjectKey: "TEST", projectPath: "flightjs/flight", 
    usersMapping: [
      {jiraAccountId: "aaaa", gitlabId: 5}, 
      {jiraAccountId: "bbb", gitlabId: 7},
      {jiraAccountId: "ccc", gitlabId: null},
      {jiraAccountId: "ddd"}
    ]}) {
    errors
    jiraImport {
      jiraProjectKey
    }
  }
}

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.

#214808 (closed)