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)
What if self.currentAccount() *always* returns a model, it's fields are accessed via `get`, and `get` returns null/undefined if there is no available current account? That would eliminate the worry I have here, plus simplify [the code](https://github.com/mozilla/fxa-content-server/pull/1733/files#diff-1e7023ec1cef9d94ba560d6b63545743R34) below [in confirm](https://github.com/mozilla/fxa-content-server/pull/1733/files#diff-1e7023ec1cef9d94ba560d6b63545743R45).