Implement Notification#card

Models now know how to find their cards, rather than trying to
implement this logic in a view helper.

app/helpers/notifications_helper.rb

@@ -66,15 +66,10 @@ module NotificationsHelper
     end
 
     def notification_closed?(notification)
-      card_for_notification(notification).closed?
+      notification.card.closed?
     end
 
     def notification_color(notification)
-      card_for_notification(notification).color
-    end
-
-    def card_for_notification(notification)
-      eventable = notification.source.eventable
-      eventable.respond_to?(:card) ? eventable.card : eventable
+      notification.card.color
     end
 end

app/models/card.rb

@@ -31,6 +31,10 @@ class Card < ApplicationRecord
     [ super, collection.name ].compact.join("/")
   end
 
+  def card
+    self
+  end
+
   private
     def set_default_title
       self.title = "Untitled" if title.blank?

app/models/event.rb

@@ -9,6 +9,8 @@ class Event < ApplicationRecord
 
   after_create -> { eventable.event_was_created(self) }
 
+  delegate :card, to: :eventable
+
   def action
     super.inquiry
   end

app/models/mention.rb

@@ -7,6 +7,8 @@ class Mention < ApplicationRecord
 
   after_create_commit :watch_source_by_mentionee
 
+  delegate :card, to: :source
+
   def self_mention?
     mentioner == mentionee
   end

app/models/notification.rb

@@ -10,6 +10,7 @@ class Notification < ApplicationRecord
   after_create_commit :broadcast_unread
 
   delegate :notifiable_target, to: :source
+  delegate :card, to: :source
 
   def self.read_all
     update!(read_at: Time.current)