Run the thumbnail generator after a file import

https://redmine.teklia.com/issues/10879

Requires #1914 (closed), arkindex/workers/import/file#12 (closed)

In file imports, the thumbnail generator should be run right after the file import worker so that thumbnails can generated immediately for all folders. The task should run with an environment variable set to point it to the list of folders: TASK_ELEMENTS=/data/import_files/folders.json