Disable FPC when Turpentine is Enabled

The code in this pull request does the following to the Cache Management page:
* It automatically disables the FPC cache if the Turpentine caches are switched from "disabled" to "enabled".
* It forces the FPC cache to remain disabled my removing the checkbox next to it while the Turpentine caches are enabled. The only way to re-enable FPC is to first disable the Turpentine caches.

We plan on rolling out these changes to all clients that use Turpentine because their admins sometimes get confused when they see a disabled item in the Cache Management page and they try to re-enable it. Which makes the site blow up basically because [FPC doesn't play nice with Turpentine](https://github.com/nexcess/magento-turpentine/issues/86).