[keyserver] factor out identity auth and saveIdentityInfo from verifyUserLoggedIn

Summary:
we'll use the authAndSaveIdentityInfo function in keyserver.js where we currently call verifyUserLoggedIn. this is because we care about how the identity info was fetched. if it was fetched directly from the identity service, there's no need to call syncPlatformDetails. however, if it fetched from mariadb, we should attempt to sync.

Depends on D13665

Test Plan:
cleared access token from metadata table and logged out of keyserver. ran yarn dev and was logged back in. on subsequent yarn dev, auth metadata was fetched from mariadb

tested more in next diff

Reviewers: ashoat

Reviewed By: ashoat

Subscribers: tomek

Differential Revision: https://phab.comm.dev/D13666