[server] Updated createAccount to call createPrivateThread

Summary: Added createPrivateThread function to accouunt-creator.

Test Plan: Tested both individually and in combitation with the other code. I created new accounts to make sure the thread was being properly created, I also checked phpMyAdmin to make sure the squadBot had a -1 role, and for permissions had NULL. I made sure creating new threads with users still work on old and new accounts, and I also made sure threads still work on old accounts.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D649