Add assignee field to issues CSV import

Improving https://docs.gitlab.com/ee/user/project/issues/csv_import.html

GitLab would search for assignee or assignees in the CSV header when doing the import.

GitLab would look at that field and look for one more email addresses there, and match them to email addresses in GitLab. If there's a match, then that person is assigned to the issue.

For GitLab Core, it would only assign one person. For GitLab Starter and above, it would assign as many matches as possible.