Fix for users who are unable to sign out.

When upgrading localStorage from loggedIn to use the site namespace, I failed to remove the old loggedIn namespace, making it so users would sign out, refresh the page, and be signed in again.

fixes #3386
Add a test to ensure re-invoking upgradeLoggedInInfo does not throw an error.
Merge pull request #3398 from mozilla/issue_3386_cannot_sign_out

Fix for users who are unable to sign out.