[server] Generate updates in replies count migration script

Test Plan: I set replies_count to 0 for all the threads, logged out and in on the device and noticed that 1 reply was displayed for every sidebar. Then I ran the script and the count was immediately increased to the correct value.

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, atul, subnub, karol-bisztyga

Differential Revision: https://phabricator.ashoat.com/D702