Improvements to Fundraiser Upsell admin interface

This branch helps improve the node form settings for the Fundraiser Upsell module. Here is what they do:

* Add field descriptions to help warn an administrator not to combine Upsell with a recurring form or non-recurring-enabled gateway;
* Add code to the validation hook to prevent an admin from saving a donation form with Upsell + recurring or Upsell + a non-recurring-enabled gateway
* Show/hide all Upsell configuration fields based on checking or un-checking the "Upsell enabled" checkbox. This is just for visual clarity/simplicity.