[server] Moved the private threads description to the thread-creator file.

Summary: Moved the private threads description from the 3 seperate files (account-creator, thread-creator, and create-private-threads) into thread-creator with an export instead.

Test Plan: Made sure squadbot has the right message when creating an account, made sure that private threads still have the correct desciption, and also made sure the create-private-threads script still sets the right description.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D730