Creating an account now creates a private thread instead. With a title and desciption of private.

Summary: Made it so creating an account now creates a private thread instead. This private thread has a title and desciption of private thread. Also made it so the createThread function in thread-creator accepts PRIVATE threads without returning invalid_parameters.

Test Plan: Tested indivdually and in combination with the other code required to create/use private threads. Tested by creating new accounts, clicking on the private thread, going into settings of the private thread. Sending messages to the private thread. And also making sure the previous thread types still work by creating other accounts and messaging each other. Also made sure that older accounts before private threads still work properly, both with the old thread with just their name, and with threads with other users.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D625