Refactor AuthenticationHelper and use Account for storing cookie info

This fixes #110 (webview to display profile on site) *again*. We had a problem with it losing the current cookie and failing authentication.

This is a pretty disruptive PR though, as I ended up doing some fairly valuable, but again disruptive, things. I think they're all worth doing.

* Moved all account access into the AuthenticationHelper class, so things get done the same wherever they're done from.
* Moved the storage of the cookie into the Account object; it turned out that was a pretty easy thing to do.

I don't think we have to retest on lower APIs, but unfortunately this should get another whole round on:

* Login
* Delete account in settings while app is up
* Change password on site while app is running
* Change login credentials while app is running (going to Menu->Account)
* After those, try to access the "view on site" - both the view profile and the leave feedback should result the same.

I'm pretty sure it's a step forward.