Use Field::RichText type for ActionText has_rich_text associations

- detects ActionText rich text associations based on class name and
  attribution name and if it's a match, returns the built-in
  Field::RichText type

Author: smartygus

lib/generators/administrate/dashboard/dashboard_generator.rb

@@ -128,7 +128,9 @@ def column_types(attr)
 
       def association_type(attribute)
         relationship = klass.reflections[attribute.to_s]
-        if relationship.has_one?
+        if relationship.name.start_with?('rich_text_') && relationship.options[:class_name] && relationship.options[:class_name].start_with?('ActionText::')
+          "Field::RichText"
+        elsif relationship.has_one?
           "Field::HasOne"
         elsif relationship.collection?
           "Field::HasMany"