[web] Use real thread creator

Test Plan:
Open pending sidebar and send a message: new thread should be created.
Open existing thread and send a message: new thread must not be created.
Check if creating sidebars on native still works.

Reviewers: ashoat

Reviewed By: ashoat

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

Differential Revision: https://phabricator.ashoat.com/D748