Add support for new document page structure

Documents now have their own dedicated DocumentPage
submodel. The old DocumentPage is now called DocumentVersionPage.
This allows mappings between document pages and document version
pages, allowing renumbering, appending pages.
DocumentPages have a content_object to map them to any other
object. For now they only map to DocumentVersionPages.
New option added to the version upload form to append the
pages of the new version.
A new view was added to just append new pages with wraps the
new document version upload form and hides the append pages
checkbox set to True.
Add a new action, reset_pages to reset the pages of the
document to those of the latest version.

Missing: appending tests, checks for proper content_object in OCR and
document parsing.

Author: Roberto Rosario <roberto.rosario@mayan-edms.com>
Date:   Thu Oct 11 12:00:25 2019 -0400

mayan/apps/document_parsing/api_views.py

@@ -8,7 +8,7 @@ from rest_framework.response import Response
 from mayan.apps.documents.models import Document
 from mayan.apps.rest_api.permissions import MayanPermission
 
-from .models import DocumentPageContent
+from .models import DocumentVersionPageContent
 from .permissions import permission_content_view
 from .serializers import DocumentPageContentSerializer
 
@@ -41,8 +41,8 @@ class APIDocumentPageContentView(generics.RetrieveAPIView):
 
         try:
             content = instance.content
-        except DocumentPageContent.DoesNotExist:
-            content = DocumentPageContent.objects.none()
+        except DocumentVersionPageContent.DoesNotExist:
+            content = DocumentVersionPageContent.objects.none()
 
         serializer = self.get_serializer(content)
         return Response(serializer.data)