R1:0e7a5623e32c

Allow task statuses to "lock" them, preventing additional comments and interactions

Summary:
Ref T12335. See that task for discussion. Here are the behavioral changes:

- Statuses can be flagged with `locked`, which means that tasks in that status are locked to further discussion and interaction.
- A new "CAN_INTERACT" permission facilitates this. For most objects, "CAN_INTERACT" is just the same as "CAN_VIEW".
- For tasks, "CAN_INTERACT" is everyone if the status is a normal status, and no one if…