[server] Save initial message author as a prop of create sidebar message

Summary: We need this to be able to include initial message author's name in notification and robotext

Test Plan: Create a message in db and verify using console log that frontend received the message with initial message author

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub

Differential Revision: https://phabricator.ashoat.com/D606