Fronts: new importance settings

Editors can put as many stories in a bucket as they want to. This basically changes the whole template logic behind curated fronts.

See https://docs.google.com/a/guardian.co.uk/document/d/1l1Qs48T43e4bcGFwZvciiN4fYYmyPZKZB3GJTshLvFU/edit for more info.

![fronts-editor-new-spec](https://cloud.githubusercontent.com/assets/85783/2819151/8d03d0fe-ceeb-11e3-8f8f-78b656174e89.png)

![ ](http://media.giphy.com/media/bEE4kvmWjyMXm/giphy.gif)