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)