Consolidate use of magical cookie name strings

Summary: Ref T4339. We have more magical cookie names than we should, move them all to a central location.

Test Plan: Registered, logged in, linked account, logged out. See inlines.

Reviewers: btrahan

Reviewed By: btrahan

CC: aran

Maniphest Tasks: T4339

Differential Revision: https://secure.phabricator.com/D8041