(core) Fix org-session mismatch in getBootKeySessionId

Summary:
This fixes a bug during the authentication setup step on fresh Grist installations where
applying getgrist.com authentication would sign out the user, even when their only session
was from a boot key.

Test Plan: Server tests

Reviewers: paulfitz, Spoffy

Reviewed By: paulfitz, Spoffy

Subscribers: Spoffy, paulfitz

Differential Revision: https://phab.getgrist.com/D5058