[server][lib] Update user store after thread creation

Summary: Return user infos from thread creation and reduce it just like when joining a thread

Test Plan: I opened a pending thread and sent a message. The created thread had a correct name, thread creation message contained correct user name and name was displayed in thread settings

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D490