Replace "web" and "conduit" magic session strings with constants
Summary: Ref T4310. Ref T3720. We use bare strings to refer to session types in several places right now; use constants instead.
Test Plan: grep; logged out; logged in; ran Conduit commands.
Reviewed By: btrahan
Maniphest Tasks: T4310, T3720
Differential Revision: https://secure.phabricator.com/D7963