Use backbone and nunjucks

TL; DR: Using Backbone  + nunjucks is a better fit to our use-case.

I decided to take a fresh look at what we could use to do this app, before painting us into a corner with a bad choice. In the end I've decided to *not* use Ember (or Angular - I took another look at this). I've switched to backbone, because it's fairly small, concise and it has no template solution baked in so we can use nunjucks.

The template choice is quite a big part of this. Nunjucks is already in-use in fireplace, familiar - and it will work nicely with i18n-abide for l10n.

Another part of this was looking at what we actually need. Our reliance on models is going to be minimal, possibly a User model for state management. It felt to me that Ember/Angular et al are all geared towards much bigger apps. Not to say Backbone can't do that too but if feels like a much better fit so far as I can use what I need and ignore what I don't need more easily than try and fit our use-case into something that is geared towards larger apps.