Port thumbnail generation task

Refs https://redmine.teklia.com/issues/6067

Easier after workers/base-worker#290 (closed)

We need to convert the code for thumbnails generation from arkindex tasks as a standalone worker.

It will use base-worker, and be implemented as an ElementsWorker.

There will be no handling of local JSON files anymore, instead relying on process_element to build a thumbnail for folders with 1 or more children images.

You can probably use generic methods from base worker for some API calls instead of using directly the arkindex API client.

Workflow:

• Process folders (skip element with a message, if element.folder is False)
• Retrieve first elements (port _list_folder_elements)
• Retrieve their images (port get_first_images)
• Build the thumbnail (port generate_thumbnail)
• Upload it (use the helper from base-worker)

Configuration parameters:

• width: 900, used as parameter in get_first_images

User configuration parameters:

• first_n: number of images to use to build the thumbnails, int, defaults to 3.

Use arkindex_worker.image.open_image instead of porting the method.