Improve CLI script for account creation and document account/reg setup process

Summary:
There was an old "create_user.php" script but it really was only useful for
creating agents. Provide a more user-friendly script for creating the first
account.

Depends on D278.

Test Plan:
Used 'accountadmin' to create and edit accounts. Read documentation.

Reviewed By: tuomaspelkonen
Reviewers: jungejason, tuomaspelkonen, aran
CC: ccheever, aran, tuomaspelkonen
Differential Revision: 279