Add a modern "user.edit" API method for users

Summary:
Depends on D19576. Ref T13164. See PHI642. This adds an EditEngine for users and a user.edit modern API method.

For now, all it supports is editing real name, blurb, title, and icon (same as "Edit Profile" from the UI).

Test Plan:

• Edited my stuff via the new API method.
• Tried to edit another user, got rejected by policies.
• Tried to create a user, got rejected by policies.

Reviewers: amckinley

Reviewed By: amckinley

Maniphest Tasks: T13164

Differential Revision: https://secure.phabricator.com/D19577