Validate CAP against the spec

This gives us some extra confidence that there aren’t any problems with
the data we’re getting from the other service. It doesn’t address any
specific problems we’ve seen, rather it seems like a sensible precaution
to take.

app/xml_schemas/__init__.py

@@ -0,0 +1,19 @@
+from lxml import etree
+from pathlib import Path
+
+
+def validate_xml(document, schema_file_name):
+
+    path = Path(__file__).resolve().parent / schema_file_name
+    contents = path.read_text()
+
+    schema_root = etree.XML(contents.encode('utf-8'))
+    schema = etree.XMLSchema(schema_root)
+    parser = etree.XMLParser(schema=schema)
+
+    try:
+        etree.fromstring(document, parser)
+    except etree.XMLSyntaxError:
+        return False
+
+    return True