Start user accounts ID with a large value

Summary:
Reference: https://secure.phabricator.com/book/phabflavor/article/things_you_should_do_now/

Fixes T830.

Test Plan:

• php artisan migrate
• Ensure auto increment value is now gigantic
• Rollback
• Ensure auto increment value is the last user id value + 1.

Reviewers: dereckson

Maniphest Tasks: T830

Differential Revision: http://devcentral.nasqueron.org/D379