Add auth, saving & loading apps

[don't pull yet]

- [x] bring back Persona, but isolate that stuff in a Polymer element called <user-state> (happy to consider other names).
- [x] remove Persona branding cause it makes my design-skills break
- [x] refactor mongo stuff to have two models, one for components and one for apps
- [ ] refactor components thing to use auth so that components aren't global, but per user
- [ ] add tests