[keyserver] Only set sidebarCreation flag for the first message in the sidebar

Summary:
When I say "first message" in the commit title, I really mean the third message, as each sidebar starts with a SIDEBAR_SOURCE and then a CREATE_SIDEBAR.

Check out the code comment for an explanation as to why we're doing this.

Test Plan: Force sidebarCreation to true on the client for all text messages, and confirm that it is ignored when invalid

Reviewers: atul

Reviewed By: atul

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D6932