[keyserver] Add avatar column to threads table of serverDB

Summary: Based off of the avatar column that was already created in the users table.

Test Plan:
Tested the migration query in MariaDB query console.

Copy/pasted the query in setup-db from users table.

Reviewers: ginsu

Reviewed By: ginsu

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D7098