[keyserver] dedup username db queries

Summary: we have similar SQL queries in search/users.js and creators/account-creator.js. we should just use searchForUser everywhere

Test Plan: tried creating a user with a username that already exists. it failed, as expected

Reviewers: atul, ashoat

Reviewed By: ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8735