Add milestone field to issues CSV import

Problem to solve

Currently, the CSV Import only imports 2 fields (name, description), even though the CSV Export contains many more fields.

This issue is about importing an additional field, which is currently ignored - milestone.

Proposed solution

GitLab would search for milestones in the CSV header when doing the import.

GitLab would look at that field and do a matching of what is in that field, and to all assignable milestones to the GitLab project for that import. If there is a match, then GitLab will assign that milestone. Multiple matches (ties) are broken arbitrarily.

Documentation

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