[keyserver, lib] Set target_message for new sidebar_source messages

Summary:
issue: https://linear.app/comm/issue/ENG-3708/add-a-migration-to-populate-target-message-for-sidebar-source
We need the new sidebar_source messages to have their target_message column filled with the id of the original message.

Test Plan:
Created new sidebar, checked that the target_message was correctly filled.
Run flow in keyserver/ and lib/

Reviewers: bartek, kamil, ashoat

Reviewed By: bartek, ashoat

Subscribers: ashoat, tomek, atul

Differential Revision: https://phab.comm.dev/D7516