fix(avatars): load profile image on settings and sign in pages if available

 This refactor changes a number of things:
    * fix avatar background (fixes #1722)
    * loads profile images on settings and sign in account chooser (fixes #1727)
    * introduces User and Account models
    * imports old Session based sessions into User
    * handles direct link to settings page from RPs for a specified account (fixes #1686)
    * redirects unverified accounts that visit /settings (fixes #1716)
    * introduces a way to pass ephemeral data between views
    * removes unused avatar-change-via-URL resources/tests (fixes #1544, fixes #1618)
You could also do something like:
```js
function profileMethodDelegate(method) {
  var self = this;
  var args = [self.accessToken].concat([].slice.call(arguments, 1));
  return self.profileClient()
    .then(function (profileClient) {
       return profileClient[method].apply(profileClient, args);
    });
}

['getAvatar', 'getAvatars', 'postAvatar', 'deleteAvatar', 'uploadAvatar']
 .forEach(function (method) {
   Account.prototype[method] = _.partial(profileMethodDelegate, method);
 });
```

Not sure if that's any clearer though.