[keyserver] Modify updateThread to handle avatars

Summary: Able to send avatar setting change to keyserver and have it reflected in the DB.

Test Plan:
Able to see avatar field populated in threads table:

Able to see avatar change robotext (obviously needs to be changed):

Reviewers: ashoat, ginsu

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D7332