Add the four primary work item types to DB - issue, incident, test_case, and requirement

This change adds the 4 primary work item types: issue, incident, test_case, and requirement.

It also changes the enum Issue.issue_types into WorkItem::Type.base_types. The values stay the same, so there is no issue of compatibility.

Upcoming MRs will begin filling in the Issue.work_item_type_id