Make static assets S3 bucket name configurable

[i-like-robots]

Currently the deploy-hashed-assets task will send files from the configured local directory (./public/ by default) to a folder named hashed-assets/app-name (app-name being inferred from the application's pkg.name).

https://github.com/Financial-Times/n-heroku-tools/blob/master/tasks/deploy-hashed-assets.js#L66

With the Core UI project we intend to deploy all hashed assets to a shared folder. This is because we will be splitting our JS bundle into more granular parts with the intention that this will increase cache-ability, reduce the amount we bust the cache, and enable greater reuse of assets between our user facing applications.

[i-like-robots]

Fixed by #556 and #560