[Keyserver] Refactor identity utilities

Summary:
These are useful when interacting with identity service,
pull them out of login.js to make them more widely available.

Part of https://linear.app/comm/issue/ENG-3944

Depends on D8474

Test Plan: Launch keyserver with login workflow (missing identity info)

Reviewers: kamil, michal, ashoat

Reviewed By: kamil, ashoat

Subscribers: ashoat, tomek

Differential Revision: https://phab.comm.dev/D8475