Support downloading from s3 (and other datastores)

...po/controllers/download_controller/download_controller.go

...rollers/maintenance_controller/maintainance_controller.go

...ontrollers/preview_controller/preview_resource_handler.go

.../controllers/thumbnail_controller/thumbnail_controller.go

...ollers/thumbnail_controller/thumbnail_resource_handler.go

...a-repo/controllers/upload_controller/upload_controller.go

...turt2live/matrix-media-repo/internal_cache/media_cache.go

...urt2live/matrix-media-repo/storage/datastore/datastore.go

...live/matrix-media-repo/storage/datastore/datastore_ref.go

...matrix-media-repo/storage/datastore/ds_file/file_store.go

...ive/matrix-media-repo/storage/datastore/ds_s3/s3_store.go

...ithub.com/turt2live/matrix-media-repo/storage/ds_utils.go

...turt2live/matrix-media-repo/storage/startup_migrations.go

...github.com/turt2live/matrix-media-repo/types/datastore.go

src/github.com/turt2live/matrix-media-repo/util/random.go

...ub.com/turt2live/matrix-media-repo/util/util_exif/exif.go