Use site-helper to login on page visit

[ianb]

#2167 is fixed in 6adbfea we won't be setting the session cookie on startup.

As a result you could use Page Shot to create a shot (and create an account), then restart Firefox and lose your login cookie, and then return to a shot or /shots, and you wouldn't be logged in.

We can detect this in site-helper.js but then we have to deal with it. I'm not sure what's the best way to do that. We could trigger a login, but then the page will have been rendered as though you weren't logged in. I suppose we could change the model in-place and rerender without too much trouble.

[ianb]

Note that you might not be even registered at that moment, in which case we should probably not log you in.