[4/n] [server] rename accountUpdater --> passwordUpdater, update legacy, prepend with old

Summary: update account to password, prepend old to legacy password updater

Test Plan: reset password, confirm functionality still works

Reviewers: atul, palys-swm, ashoat

Reviewed By: ashoat

Subscribers: ashoat, Adrian, karol-bisztyga, boristopalov

Differential Revision: https://phabricator.ashoat.com/D2381