[server] Added create-private-threads script

Summary: Added create-private-threads script to convert the old private threads to the new private threads.

Test Plan:
Made sure the findThreadsToUpdate query only returns the old private threads, and made sure it does not return group chats.

Made sure the converted private threads have the correct type, permissions and description, both in native and phpMyAdmin.

Made sure you can send messages in the converted private threads.

Made sure you can still see older messages of the private threads after being converted.

Made sure normal and group chats still work and have the correct types and permissions.

Made sure when creating an account with private threads still work properly.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D722