Redirect to homepage on logout

Previously the app would redirect to the referrer page, which would
often result in an error if the previous page was for logged-in users
only. We now redirect to the homepage regardless of where the user was
when they logged out.
Save oauth credentials on every login

The Fitbit API changed recently to issue new user token/secret
credentials every time a user logs in, rather than to re-issue the same
credentials. That means that if we've stored previous oauth creds to the
database that we need to ensure the latest ones received overwrite the
previous ones.