Expose poll_maximum_options to client

At present this doesn't actually seem to be exposed to the client.

I discovered this by accident whilst trying to write my own plugin:

7b6d6b76eb/app/models/site_setting.rb (L17-L18)

plugins/poll/config/settings.yml

@@ -3,3 +3,4 @@ plugins:
     default: true
   poll_maximum_options:
     default: 20
+    client: true