Use a Many-to-Many relation between Document and OCRModel

The goal is to convert the OcrModel.document Foreign Key to an OcrModel.documents Many-to-Many relation towards Document instances.

This means several low-level changes:

• an OcrModel would then be usable across multiple documents
• a document would still be usable across multiple OcrModel instance
• the OcrModel.owner user would be the administrator of the model, no access rights could be easily extracted/aggregated from documents

A M2M "through" model named OcrModelDocument would be created, only to store the relationship between OcrModel and Document alongside a created date (it's easier to do that from the beginning than a later migration to add it).

The OcrModelDocument model would also store boolean states descritbing the interactions between an OcrModel and a Document:

• trained_on would be used when the model has been trained on the document
• executed_on would be used when the model has been executed on the document to produce results

A unique constraing on document_id and model_id would ensure than only one M2M relation is possible between a model and a document.

This kind of changes can break a lot of functionalities in small ways, so we would need to fix the behaviour of (at least) these parts:

• document creation interface
• training tasks
• access control to models and documents
• API OcrModelViewSet and its usage

At this stage, we only update the database and surrounding code, reproducing current behaviour. The resulting code should be mergeable as-is and serve as basis for features development.