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, Bob", they almost certainly mean "Alice, Bob, and the task author".

And I ended up deleting myself by accident way more often than I deleted myself on purpose -- especially with "Create Similar task", I'd sometimes delete all the CCs and delete myself by accident and then have to put myself back.

Finally, technically speaking, restoring the old behavior is kind of hard/messy and this is much easier.

Test Plan:

• Created a task.
• Was automatically added as a subscriber.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T9908

Differential Revision: https://secure.phabricator.com/D14694