Allow finders to be defined for each action.

Per #3119 this will allow one to define a class such as:

    module Finders
      class Show
        def call(params)
          # logic for finding a page
        end
      end
    end

The advantage of this is that it allows any number of custom actions
and somewhat removes the hardcoded nature of home and show.

pages/app/controllers/refinery/pages_controller.rb

@@ -68,12 +68,7 @@ def first_live_child
     end
 
     def find_page(fallback_to_404 = true)
-      @page ||= case action_name
-                when "home"
-                  Refinery::Page.find_by(link_url: '/')
-                when "show"
-                  Refinery::Page.friendly.find_by_path_or_id(params[:path], params[:id])
-                end
+      @page ||= action_page_finder.call(params) if action_has_page_finder?
       @page || (error_404 if fallback_to_404)
     end
 
@@ -89,5 +84,28 @@ def write_cache?
         cache_page(response.body, File.join('', 'refinery', 'cache', 'pages', request.path).to_s)
       end
     end
+
+    private
+    def action_has_page_finder?
+      Finders.const_defined? action_name.classify
+    end
+
+    def action_page_finder
+      Finders.const_get action_name.classify
+    end
+
+    module Finders
+      class Home
+        def self.call(_params)
+          Refinery::Page.find_by link_url: "/"
+        end
+      end
+
+      class Show
+        def self.call(params)
+          Refinery::Page.friendly.find_by_path_or_id params[:path], params[:id]
+        end
+      end
+    end
   end
 end