Add initialMessageID parameter when creating sidebar

Summary: First step of displaying message that started sidebar.

Test Plan: Make sure creating sidebars is possible only with new parameter, and all of the rest thread types do not have it

Reviewers: ashoat, KatPo

Reviewed By: ashoat

Subscribers: zrebcu411, Adrian

Differential Revision: https://phabricator.ashoat.com/D541