R1:65904d7c5137

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

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