[lib] Do not include members list in pending sidebar id

Test Plan: Create one new sidebar with only the viewer, one with two members and use thread creator to create a new thread. In all the cases check if the real thread contains the correct members.

Reviewers: palys-swm

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D917