Added PRIVATE thread type. Made the integer value 7. Also added this type to asserThreadType, NewThreadRequest, and threadTypePropType

Summary: Added PRIVATE thread type. Made the integer value 7. Also added this type to assertThreadType, NewThreadRequest, and threadTypePropType

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.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul

Differential Revision: https://phabricator.ashoat.com/D623