Add notification_methods for adding methods to the Notification model

[excid3]

This introduces a notification_methods block for Notifiers to add methods to the Noticed::Notification records. This works by adding STI to Noticed::Notifications, dynamically creating a Notification class for each Notifier and applying methods to it.

Without this feature, it is tricky to load notifications from the database and call methods on the Notifier that require the notification or recipient:

<div>
  <% @user.notifications.includes(:event).each do |notification| %>
    <%= link_to notification.event.message_for(notification)  %>
  <% end %>
</div>

class NewCommentNotifier < Noticed::Event
  # ...

  def message_for(notification)
    "#{notification.recipient.name}, you have a new comment to review"
  end
end

While this is fine, it feels incomplete.

By introducing notification_methods, we can refactor the above to this:

<div>
  <% @user.notifications.includes(:event).each do |notification| %>
    <%= link_to notification.message  %>
  <% end %>
</div>

class NewCommentNotifier < Noticed::Event
  # ...

  notification_methods do
    def message
      "#{recipient.name}, you have a new comment to review"
    end
end

This is considerably cleaner and provides a handy way of extending the Notification class while also making the methods unique to each Notifier. If we added the message method to Noticed::Notification directly, it would share that method across all Notifiers.

See #360 for the discussion leading to this.

[jon-sully]

🙌