Better loop field toggle icon

This PR updates the loop field code and design to change an individual loop field's toggle icon based on whether it's open or closed.

Open:

![cursor_and_new_campaign_ __225__ _wordpress](https://cloud.githubusercontent.com/assets/1231306/4519874/97eb897e-4cce-11e4-8132-322a290c4f4d.png)

Closed:

![cursor_and_new_campaign_ __225__ _wordpress](https://cloud.githubusercontent.com/assets/1231306/4519875/a07c203a-4cce-11e4-907c-dbaa62933ee6.png)

The up/down arrows mirror the WP meta box toggle icons.

This PR does not cover legacy styles, for non mp6-ers. It could also probably be simplified by applying the "open" class to the whole loop div, and tweaking the styles to match. I'm happy to refactor if that's desired.