Update issues table related scopes to stop using the issue_type column

Multiple scopes use the issue_type column for filtering. We need to change them to use the work_item_types table instead of the issues.issue_type column.

Some of this changes might require indexes to be replaced. The following indexes need a replacement index when working on the scope that uses them:

• index_issues_on_incident_issue_type (already replaced by index_issues_on_work_item_type_id)
• index_issues_on_project_id_and_created_at_issue_type_incident
• index_on_issues_closed_incidents_by_project_id_and_closed_at