Make user creation process simpler

Summary:
Fixes T4065. This divides user creation into separate "Standard User" and "Script/Bot" workflows which show only relevant fields and provide guidance.

This fixes the verification mess associated with script/bot users by verifying their email addresses automatically.

Test Plan:
  - Created a standard user.
  - Created a script/bot.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T4065

Differential Revision: https://secure.phabricator.com/D8674