[server] Updated thread-creator to use export instead of export default

Summary: Updated thread-creator to use export instead of export default.

Test Plan: Made new accounts to test the new private thread function. Made sure the createThread function still worked for normal threads/groups too.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D686