Set incident status to fixed when incident update status is fixed

[406-mot-acceptable]

Summary

• When an incident update has status fixed, also transition the parent incident's status column to fixed.
• Previously only the linked component pivots were reset (via Set linked components to operational when incident is fixed #333), so the parent incident stayed on its last status (e.g. investigating) and the public banner kept reporting "Some systems are experiencing issues."
• Addresses Creating an incident and then fixing it still shows "Some systems are experiencing issues." on status page. cachet#4516.

Test plan

• Added test: parent incident status transitions to fixed on a fixed update.
• Added test: parent incident status unchanged on non-fixed updates.
• Full suite: 382 passed.