[keyserver] simplify verifyUserLoggedIn

Summary:
in the next diff, i split out the auth and save parts of this function into a separate function, so rearranging the code here first

Depends on D13664

Test Plan: read the code carefully

Reviewers: ashoat, inka

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13665