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)
@zaach - it seems like `Session` is pretty thin. Think we can move the oauth stuff to be completely handled by the oauth broker and do away with the module entirely, or are we storing other stuff here?