fix(avatars): load profile image on settings and sign in pages if available

Fixes #1727

Also fixes #1722.
Closes #1544.
Closes #1613.
In the sense that each model should manage its own data, it should probably be on the `User` model.

I'd also like us to be a little more disciplined with this because bugs involving "legacy" `localStorage` state are very hard to test and reproduce. Here's something like what I'd like to see: https://github.com/mozilla/fxa-content-server/issues/1833