Improve docs management and add invoices repo (#51)

* Create a repository app for documents, first step

* Step two (broken)

* Complete the implementation of document repository and add invoices

* Add support for invoices

* Update submodules

ram/roster/models.py

@@ -8,7 +8,7 @@ from django.dispatch import receiver
 
 from tinymce import models as tinymce
 
-from ram.models import BaseModel, Document, Image, PropertyInstance
+from ram.models import BaseModel, Image, PropertyInstance
 from ram.utils import DeduplicatedStorage, slugify
 from ram.managers import PublicManager
 from metadata.models import (
@@ -169,20 +169,6 @@ def pre_save_internal_fields(sender, instance, *args, **kwargs):
     instance.item_number_slug = slugify(instance.item_number)
 
 
-class RollingStockDocument(Document):
-    rolling_stock = models.ForeignKey(
-        RollingStock, on_delete=models.CASCADE, related_name="document"
-    )
-
-    class Meta:
-        constraints = [
-            models.UniqueConstraint(
-                fields=["rolling_stock", "file"],
-                name="unique_stock_file"
-            )
-        ]
-
-
 def rolling_stock_image_upload(instance, filename):
     return os.path.join(
         "images",