uploads: Allow uploads to set storage class.

Uploads are well-positioned to use S3's "intelligent tiering" storage
class.  Add a setting to let uploaded files to declare their desired
storage class at upload time, and document how to move existing files
to the same storage class.

zproject/prod_settings_template.py

@@ -768,6 +768,7 @@ LOCAL_UPLOADS_DIR = "/home/zulip/uploads"
 # S3_REGION = None
 # S3_ENDPOINT_URL = None
 # S3_SKIP_PROXY = True
+# S3_UPLOADS_STORAGE_CLASS = "STANDARD"
 
 ## Maximum allowed size of uploaded files, in megabytes.  This value is
 ## capped at 80MB in the nginx configuration, because the file upload