Put API keys on service model

Similar to how we put templates on the service model, it means less
logic needs to happen in the view code.

app/templates/views/api/keys.html

@@ -37,7 +37,7 @@
 
   <div class="body-copy-table">
     {% call(item, row_number) list_table(
-      keys,
+      current_service.api_keys,
       empty_message="You haven’t created any API keys yet",
       caption="API keys",
       caption_visible=false,