NGA smart banner upsell.

This introduces a JS module to display a call-to-action at the top of the website to up-sell the new next-gen apps when they are released next week.

*Notes:*
- It is behind a switch and will not be turned on until next Friday 30th
- Only targets iOS and Android users
- Design has been supplied by marketing and converted into HTML/CSS for accessibility reasons.
- Not using native iOS meta tag for various reasons
-- Uses current app store rating (poor)
-- Does not enable conversion tracking

*Rules:*
- Only display if you have visited 4 times in the last month
- Only show 3 times
- Persist decision if closed

### Screenshots

#### iOS
![google chrome](https://cloud.githubusercontent.com/assets/80089/3052120/ee1c091c-e195-11e3-8074-4ea43cccdb12.png)

#### Android
![google chrome](https://cloud.githubusercontent.com/assets/80089/3052121/f6e77612-e195-11e3-9aac-465003db8362.png)