Make the client authoritative for "Save" actions

Summary:
Ref T13559. When you click "Save" on an inline comment and it's empty, we may actually delete the comment.

Currently, the client and server both make decisions about whether the comment should be deleted. These decisions may not agree, causing the client state to fall out of sync.

Make the client authoritative about whether it wants to handle the user clicking the "Save" button as an intent to save or an intent to delete.

Test Plan: Saved empty and nonempty inlines. See followup changes.

Maniphest Tasks: T13559

Differential Revision: https://secure.phabricator.com/D21648