Add file upload support to Cloud

Plotly Cloud has users upload all app files for every revision. However, sometimes app contains files that don’t change from revision to revision, or files which are too large and should be separated from app source code.

We would allow users to upload those files separately to their project and read from those files using a consistent path in their app. You could also write to this file store from your app if you want a persistent state from revision to revision.

We would want to allow larger file sizes (e.g. up to 2gb) for each file uploaded.