[lib] Reassign messages from pending to real thread

Test Plan:
Tested together with D842 and D844.
Open panding sidebar as one user, send failed message, create real sidebar from pending as a second user and check if the failed message was still there on 1st user's list.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D926