Add gitlab username to start jira import

What does this MR do?

It adds gitlab_username as alternartive to gitlab_id to usersMapping argument to jiraImportStart mutation.

We have decided that in the first iteration users importing data from Jira will have to input a gitlab username to a simple text field. The reason for that is that creating autocomplete component on the form is too complex.

We don't dismiss the idea of autocomplete, it will be in the next iteration. Therefore gitlab_id option is kept in the mutation & service.

Example

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

Screenshots

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 to !33320 (merged)