Allow notifications with explicit/unexistent links

Notifications usually link to the associated notifiable, but the new
AdminNotifications have a link attribute that may be empty or contain an
external or internal url.

app/controllers/notifications_controller.rb

@@ -43,9 +43,13 @@ class NotificationsController < ApplicationController
         budget_investment_path @notification.linkable_resource.budget, @notification.linkable_resource
       when "Topic"
         community_topic_path @notification.linkable_resource.community, @notification.linkable_resource
+      else
+        if @notification.linkable_resource.is_a?(AdminNotification)
+          @notification.linkable_resource.link || notifications_path
         else
           url_for @notification.linkable_resource
         end
       end
+    end
 
 end

app/models/notification.rb

@@ -53,9 +53,19 @@ class Notification < ActiveRecord::Base
       "proposal_notification"
     when "Comment"
       "replies_to"
+    when "AdminNotification"
+      nil
     else
       "comments_on"
     end
   end
 
+  def link
+    if notifiable.is_a?(AdminNotification) && notifiable.link.blank?
+      nil
+    else
+      self
+    end
+  end
+
 end