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 ephemeral data is important for Sync sign up, sign in, and reset password flows in particular. In order to hit this path to `confirm` you'd have to navigate to a settings page manually (after a sign up or sign in) in which case the account data stored in memory would already be lost. This would be hard to do from `about:accounts`, aside from QA poking around, so I'd say it's okay.