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.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4310, T3720

Differential Revision: https://secure.phabricator.com/D7963