Allow links from work items to incidents

In the Linked Items widget from a work item, such as a task, incidents appear in the "Add existing" list, but trying to add the item results in an error: #84 cannot be added: tasks cannot be related to incidents. The backend also returns null in the GraphQL response data.workItemCreate.workItem even though the incident was created.

This also prevents incidents from being related via "New related item" — more context and will prevent work item issues from being linked to incidents, which is a parity feature.

All work item types — and legacy issue types for items not yet migrated — should be available to link via linked items.