Consolidate user editing code

Summary:

• We currently have some bugs in account creation due to nontransactional user/email editing.
  • We save $user, then try to save $email. This may fail for various reasons, commonly because the email isn't unique.
  • This leaves us with a $user with no email.
• Also, logging of edits is somewhat inconsistent across various edit mechanisms.
• Move all editing to a PhabricatorUserEditor class.
• Handle some broken-data cases more gracefully.

Test Plan:

• Created and edited a user with accountadmin.
• Created a user with add_user.php
• Created and edited a user with People editor.
• Created a user with OAuth.
• Edited user information via Settings.
• Tried to create an OAuth user with a duplicate email address, got a proper error.
• Tried to create a user via People with a duplicate email address, got a proper error.

Reviewers: btrahan, vrana, jungejason

Reviewed By: btrahan

CC: tberman, aran

Maniphest Tasks: T1184

Differential Revision: https://secure.phabricator.com/D2569