Add general support for image thumbnails & media serving

[mihow]

The implementation details for this are TBD. There are common strategies with Django, but we have a unique situation where the source images are stored in external storages that are unique to each Deployment (vs. a global storage backend for Django). One option is to use the Prism API-based service that Fieldguide.ai uses. Or something else like it.

Issues related to presigned S3 urls for private buckets:

• Cache presigned_url method for the same amount of time as timeout
• Add "use presigned urls" to UI
• Session detail view needs to get presigned URLs for all captures on one page response! Can we do that from the frontend instead? Actually won't be necessary once we are thumbnailing original images
• Review if we need Whitenoise for serving images, or if we can use minio for manually uploaded captures, and other media (user icons). Can we remove the need for a django alias in /etc/hosts locally as added in Update README for local backend setup #641 ?