Allow adding broadcast templates

At the moment the page is the same as for text message templates,
except:
- different H1
- no guidance about personalisation, links, etc (until we decide how
  these should work)

For now you won’t be able to really create a broadcast template, because
the API doesn’t support it (the API will respond with a 400). But that’s
OK because no real services have the broadcast permission yet.

This required a bit of refactoring of how we check which template types
a service can use, because there were some hard-coded assumptions about
emails and text messages.

app/models/service.py

@@ -54,6 +54,7 @@ class Service(JSONModel):
         'email',
         'sms',
         'letter',
+        'broadcast',
     )
 
     ALL_PERMISSIONS = TEMPLATE_TYPES + (