Replace `issue_type` with a `work_item_types` table

Steps

• create work_item_types table - !55705 (merged)
• add work_item_type_id to issues table - !55705 (merged)
• create initial base types in table - issue, incident, test_case, requirement - !67434 (merged)
• make sure new issues are created with both an issue_type and a work_item_type_id - !67638 (merged)
• populate each issue with corresponding work_item_type_id related to it's issue_type - #338004 (closed)
• replace use of issue_type with work_item_type_id throughout code - #338005 (closed)

Proposed schema changes

Note: the issue_type on the issues table is mostly for the transition. If it has any performance value (such as reducing a query to the type table), then we might consider kepping it. Otherwise, the plan is to remove it and rely only on the type field in the work_item_types table.