R1:a1ccee8c242e

Implicitly subscribe task author when they create a task

Summary:
Ref T9908. This is a behavioral change:

- Old behavior: "Subscribers" field is default-populated with author.
- New behavior: this transaction is just created no matter what.

The new behavior is much easier to make work with form defaults, hidden fields, etc. For example, on the "Create Bug" form, I've hidden "Subscribers", but I still want the author to be subscribed.

And if a user sets the default value of "Subscribers:" to "Alice…