Add rattail provider for NationBuilder integration

and use it to generate NB URLs

rattail_nationbuilder/nationbuilder/util.py

@@ -24,8 +24,14 @@
 NationBuilder utils
 """
 
+import warnings
+
 
 def get_nationbuilder_url(config):
-    url = config.get('nationbuilder', 'url')
-    if url:
-        return url.rstrip('/')
+    warnings.warn("get_nationbuilder_url() function is deprecated; "
+                  "please use nationbuilder_handler.get_url() instead",
+                  DeprecationWarning, stacklevel=2)
+
+    app = config.get_app()
+    nationbuilder = app.get_nationbuilder_handler()
+    return nationbuilder.get_url()