Admin and disabled flags for users

Summary:
Provide an "isAdmin" flag for users, to designate administrative users.

Restore the account editing interface and allow it to set role flags and reset
passwords.

Provide an "isDisabled" flag for users and shut down all system access for them.

Test Plan:
Created "admin" and "disabled" users. Did administrative things with the admin
user. Tried to do stuff with the disabled user and was rebuffed. Tried to access
administrative interfaces with a normal non-admin user and was denied.

Reviewed By: aran
Reviewers: tuomaspelkonen, jungejason, aran
CC: ccheever, aran
Differential Revision: 278