Add validation to prevent blank area names

Now that these are used for display on gov.uk/alerts we need to
make sure the data is being set properly. We've already found an
example where it wasn't [1]. We validate external broadcasts in
two stages: with the official CAP XML schema [2] and then again
with our own, more specific schema for the converted JSON. Since
this validation is a custom requirement I've made it part of the
JSON schema. Note that jsonschema recommends avoiding metachars
like "\w" since they're not supported by all implementations [3].

I've tested the new validation manually and it works as expected
by disallowing e.g. "  " but still alowing "foo" and "foo bar".

[1]: https://www.notifications.service.gov.uk/services/120107d0-d99a-4c42-8b70-f37d2f28879b/rejected-alerts/d6e0c70e-60f6-4422-8589-2a2d159c63f2
[2]: 81a25ff1ef/app/xml_schemas/CAP-v1.2.xsd
[3]: http://json-schema.org/understanding-json-schema/reference/regular_expressions.html

tests/app/v2/broadcast/sample_cap_xml_documents.py

@@ -1,3 +1,5 @@
+import re
+
 WAINFLEET = """
     <alert xmlns="urn:oasis:names:tc:emergency:cap:1.2">
         <identifier>50385fcb0ab7aa447bbd46d848ce8466E</identifier>
@@ -235,3 +237,4 @@ WITH_PLACEHOLDER_FOR_CONTENT = """
 
 LONG_GSM7 = WITH_PLACEHOLDER_FOR_CONTENT.format('a' * 1396)
 LONG_UCS2 = WITH_PLACEHOLDER_FOR_CONTENT.format('ŵ' * 616)
+MISSING_AREA_NAMES = re.sub("<areaDesc>.*</areaDesc>", "<areaDesc> </areaDesc>", WAINFLEET)

tests/app/v2/broadcast/test_post_broadcast.py

@@ -229,3 +229,24 @@ def test_content_too_long_returns_400(
         }],
         'status_code': 400,
     }
+
+
+def test_invalid_areas_returns_400(
+    client,
+    sample_broadcast_service
+):
+    auth_header = create_service_authorization_header(service_id=sample_broadcast_service.id)
+    response = client.post(
+        path='/v2/broadcast',
+        data=sample_cap_xml_documents.MISSING_AREA_NAMES,
+        headers=[('Content-Type', 'application/cap+xml'), auth_header],
+    )
+
+    assert json.loads(response.get_data(as_text=True)) == {
+        'errors': [{
+            'error': 'ValidationError',
+            # the blank spaces represent the blank areaDesc in the XML
+            'message': 'areas   does not match ([a-zA-Z1-9]+ )*[a-zA-Z1-9]+',
+        }],
+        'status_code': 400,
+    }