Refactor to clean contracts for task input/ouput chain

[ghost]

Currently the task input is a file. This does not work well with the bucket-idiom introduced with the API. Also, for downloaded payloads it currently blocks the interface.

We need to refactor the task (as well as web and API views) to accept a uniform payload identifier as input for a task. It then can be used to trigger further actions. For example if the payload id is bucket://1234 then a folder in the storage backend named 123 is used and analyzed, model identified, etc.. If url is specified download into a bucket should occur, etc.

This input resource identifier als allows for easy chaining of tasks. For exmaple it could first be passed as input to a task which determines that it needs to download a file first, which then is downloaded, decompressed into a bucket and the actual processing task is executed with that newly created bucket URI. etc. etc.

We need to define a standard for output format of a task, so this chaining/onion model is possible.