upload to S3 only on merge?

[mschrimpf]

currently, we give the user access to S3 so that they can upload assembly/stimulus_set directly themselves.
This potentially generates files that are later unused and unnecessarily exposes data storage.

Would it be worth to have a solution where the user side only mocks the upload and then the real upload happens upon merge?
The downside of this is that the user needs to put the raw files used for packaging somewhere we can access them so we might be back to the same problem..

(copied from brain-score/brainio_contrib#37)