R1:04f9e72cbd10

Don't subscribe bots implicitly when they act on objects, or when they are mentioned

Summary:
See PHI1098. When users comment on objects, they are automatically subscribed. And when `@alice` mentions `@bailey` on a task, that usually subscribes `@bailey`.

These rules make less sense if the user is a bot. There's generally no reason for a bot to automatically subscribe to objects it acts on (it's not going to read email and follow up later), and it can always subscribe itself pretty easily if it wants…