Refactor upload static

[jmsmkn]

Uses the async best practices we found in #4342

[jmsmkn]

There isn't a CI test for this, but running it manually works as expected.

(grand-challenge-org) ➜  app git:(async_upload_static) ✗ python manage.py upload_static --bucket grand-challenge-public
get_frame_offsets is deprecated and will be removed in v4.0
Found credentials in environment variables.
Found 500 files to upload to grand-challenge-public
Found credentials in environment variables.
Uploaded s3://grand-challenge-public/unknown/images/Powered-By_logo-stack_RGB.png
...
Uploaded s3://grand-challenge-public/unknown/vendored/dcmjs/build/dcmjs.js.map
Successfully uploaded 500 files to grand-challenge-public