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)
We should really fix [this bug](https://github.com/mozilla/fxa-content-server/issues/1523) in context so these three lines can be moved to `context`