Elastic: regenerate picture URL before inserting into ElasticSearch

This ensures the thumbnail URL is public so that it won't expire.
Since this now requires API calls to Google, I've increased the number of
parallel threads used for indexing, since they'll be waiting for network
I/O more.

pillar/api/file_storage/__init__.py

@@ -338,7 +338,7 @@ def process_file(bucket: Bucket,
                     file_id, status, r)
 
 
-def generate_link(backend, file_path: str, project_id=None, is_public=False) -> str:
+def generate_link(backend, file_path: str, project_id: str=None, is_public=False) -> str:
     """Hook to check the backend of a file resource, to build an appropriate link
     that can be used by the client to retrieve the actual file.
     """