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)
A slight inconsistency is we set the current account & update the session data [from fxa-client](https://github.com/mozilla/fxa-content-server/pull/1733/files#diff-341971be75d35dbd724949cda527a0efR121), but clear the current account [in views/settings.js](https://github.com/mozilla/fxa-content-server/pull/1733/files#diff-fb9537d2bf95cbbd0025dda8d398cd6dR61). I almost feel like there should be a middleman between a view and the fxa-client that can handle the fxaClient, user, and Session. It seems like business logic that neither the views nor the fxa-client need to worry about.