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)
The [broker also](https://github.com/mozilla/fxa-content-server/blob/04b372d2b9363e5bc6f869247e790d17a27bea67/app/scripts/models/auth_brokers/base.js#L44) takes care of this and is overridden by the [FxDesktopBroker](https://github.com/mozilla/fxa-content-server/blob/04b372d2b9363e5bc6f869247e790d17a27bea67/app/scripts/models/auth_brokers/fx-desktop.js#L54), should we consolidate to reduce the chance for confusion?