Build a management command that imports a list of images and build elements

The new command import_image_list should take a few parameters:

• --image-list is a path towards a list of relative image paths (relative to a IIIF path)
• --iiif-server is the IIIF server base image url
• --iiif-prefix is the optional IIIF path
• --corpus is the required ID of a corpus
• --folder-type is the slug of a type (default to folder)
• --page-type is the slug of a type (default to page)

Workflow is :

• check corpus & types exist
• check IIIF server exists
• read each line of the image list file:
  • get or create an image on the image server (image path is : urljoin(iiif-prefix, image path from line)
  • get or create the folder hierarchy to match the image path
  • get or create a page element for the image

To summarize, if the file contains:

path/to/imageA.jpg
path/to/imageB.jpg

We should end up with 2 images, in a folder to, itself in a top folder path.

Note: it's certainly a lot more performant to first read the whole page file, and build the whole folder hierarchy.