Locking down access to your domain

The new flex environment let's you restrict authentication to users within your domain, but it's now clear how that should work for apps like this that runs on the older version of App Engine, so I aded a few lines just inside `SummaryPage` and `UserPage`
```py
user_email = self.request.get('u', _current_user_email())

# Reject all but domain users
user_domain = user_email.split('@')[-1]
app_settings = models.AppSettings.get()
if user_domain not in app_settings.domains:
self.redirect(users.create_logout_url('/'))
return
```

Start by locating the SummaryPage and UserPage handlers and reading how the current user and AppSettings.domains are used. Confirm the expected behavior for users outside the configured domains, including logout and redirect, and verify that both handlers apply the same restriction.

Written by the indexing model from the issue text.