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/templates/components/message-count-label.html

@@ -24,6 +24,12 @@
     {%- else -%}
       letters
     {%- endif -%}
+  {%- elif template_type == 'broadcast' -%}
+    {%- if count == 1 -%}
+      broadcast
+    {%- else -%}
+      broadcasts
+    {%- endif -%}
   {%- endif %} {{ suffix }}
 {%- endmacro %}