[keyserver] Deduplicate validators in activity-responders.js

Summary:
Noticed some duplication for ActivityUpdate validators, so decided to address before starting my stack.

Depends on D12359

Test Plan: Careful code inspection of the replaced validators to make sure they exactly match. Also Flow

Reviewers: tomek

Reviewed By: tomek

Differential Revision: https://phab.comm.dev/D12360