[server] Added the create sidebar permission to private threads.

Summary: Aded the create sidebars permission inside of getRolePermissionBlobsForChat.

Test Plan: Created new users and cheked if they can create sidebars on their messages. Made sure they can send messages in the sidebar. Made sure normal threads still worked correctly.

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D711