Update reference to images in index

[mpgirro]

Now that we store images in the backend, we need to update the image fields in the IndexDoc entries. Now they hold the URLs to the images, but they should have the ID of the image in the backend instead

[mpgirro]

The question is how to do this? We must first search for the respective IndexDocs, and then update the these documents. In a reverse index, this is not as trivial as it would in a database.

[mpgirro]

Seems like no complex logic is needed. Solr supports respective operations, see https://lucene.apache.org/solr/guide/7_5/updating-parts-of-documents.html

Guess we'll merely need an Atomic Update