Adds the stored source to the exports and api.

app/apps/api/serializers.py

@@ -133,7 +133,8 @@ class PartSerializer(serializers.ModelSerializer):
             'workflow',
             'order',
             'recoverable',
-            'transcription_progress'
+            'transcription_progress',
+            'source'
         )
 
     def create(self, data):

app/escriptorium/templates/export/alto.xml

 {% load export_tags %}<?xml version="1.0" encoding="UTF-8"?>
 <alto xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance"
       xmlns="http://www.loc.gov/standards/alto/ns-v4#"
-      xsi:schemaLocation="http://www.loc.gov/standards/alto/ns-v4# https://gitlab.inria.fr/scripta/escriptorium/-/raw/develop/app/escriptorium/static/alto-4-1-baselines.xsd">
+      xsi:schemaLocation="http://www.loc.gov/standards/alto/ns-v4# http://www.loc.gov/standards/alto/v4/alto-4-2.xsd">
   <Description>
     <MeasurementUnit>pixel</MeasurementUnit>
     <sourceImageInformation>
       <fileName>{{ part.filename }}</fileName>
+      {% if part.source %}<fileIdentifier>{{ part.source }}</fileIdentifier>{% endif %}
     </sourceImageInformation>
   </Description>
   {% if valid_block_types or valid_line_types %}

app/escriptorium/templates/export/pagexml.xml

 {% load export_tags %}<?xml version="1.0" encoding="UTF-8"  standalone="yes"?>
 <PcGts xmlns="http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xsi:schemaLocation="http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15 http://schema.primaresearch.org/PAGE/gts/pagecontent/2019-07-15/pagecontent.xsd">
-  <Metadata>
+  <Metadata{% if part.source %} externalRef="{{part.source}}"{% endif %}>
 	<Creator>escriptorium</Creator>
 	<Created>{% current_time %}</Created>
-    <LastChange>{% current_time %}</LastChange>
+        <LastChange>{% current_time %}</LastChange>
+
   </Metadata>
   <Page imageFilename="{{ part.filename }}" imageWidth="{{ part.image.width  }}" imageHeight="{{ part.image.height  }}">