Hide the "Author" field in Differential on the web create workflow

Summary:
Ref T13628. When creating a revision, hide the recently introduced "Author" field.

It seems exceedingly unlikely that anyone would ever want to adjust this field when creating a revision, and this workflow already has a lot of fields and complexity, so this likely adds more noise than signal on the balance.

Test Plan:

• Created revisions via copy/paste on the web workflow, no longer saw "Author".
• Used "Edit Revision" to adjust the author normally.

Maniphest Tasks: T13628

Differential Revision: https://secure.phabricator.com/D21595