In which we improve S3 support in our API

- add routes for listing, deleting and uploading S3 resources to Quests
- put S3 signing requests in URL's beneath the affected resources
- simplify S3 signing implementation in the back-end and in the front-end example code
- better docs for S3 uploads

@maiamcguinness @thcrock @malynda @wilhow
This is a pretty heavyweight import, right? Don't you just need backend.app.config?

Also, not sure if Flask works this way, but I like my config to hold variables instead of just a big lookup dict, so not only can you import them directly (from backend... import AWS_ACCESS_KEY, AWS...), but typing mistakes get caught as early as possible.