[native][web] Fill target_message column for sidebar source

Summary: issue: ENG-8891

Test Plan:
Tested that in a newly created db the target_message is filled in for sidebar source, by searching in a sidebar and getting a result for the first message of that siedbar
Tested that the migration succeeds and after it is run it is also possible to search for the first message in a sidebar

Reviewers: tomek, kamil, marcin

Reviewed By: tomek

Subscribers: ashoat

Differential Revision: https://phab.comm.dev/D12869