Close form message upon Goal Complete should be a WYSIWYG instead of Give alert

[mathetos]

Issue Overview

Give our users more flexibility to thank their donors when a campaign with a GOAL is complete by making the "Goal Achieved Message" output as a WYSIWYG instead of a Give alert.

Current Behavior

Currently we just allow them to insert a message into a textarea and it outputs like this:

Expected Behavior

Instead, let's give them a WYSIWYG so they can have full control of that content to make the THANK YOU experience much better for their donors.