REFACTOR: Improve support for consolidating notifications. (#14904)

* REFACTOR: Improve support for consolidating notifications.

Before this commit, we didn't have a single way of consolidating notifications. For notifications like group summaries, we manually removed old ones before creating a new one. On the other hand, we used an after_create callback for likes and group membership requests, which caused unnecessary work, as we need to delete the record we created to replace it with a consolidated one.

We now have all the consolidation rules centralized in a single place: the consolidation planner class. Other parts of the app looking to create a consolidable notification can do so by calling Notification#consolidate_or_save!, instead of the default Notification#create! method.

Finally, we added two more rules: one for re-using existing group summaries and another for deleting duplicated dashboard problems PMs notifications when the user is tracking the moderator's inbox. Setting the threshold to one forces the planner to apply this rule every time.

I plan to add plugin support for adding custom rules in another PR to keep this one relatively small.

* DEV: Introduces a plugin API for consolidating notifications.

This commit removes the `Notification#filter_by_consolidation_data` scope since plugins could have to define their criteria. The Plan class now receives two blocks, one to query for an already consolidated notification, which we'll try to update, and another to query for existing ones to consolidate.

It also receives a consolidation window, which accepts an ActiveSupport::Duration object, and filter notifications created since that value.

lib/plugin/instance.rb

@@ -976,6 +976,23 @@ class Plugin::Instance
     DiscoursePluginRegistry.register_reviewable_score_link({ reason: reason.to_sym, setting: setting_name }, self)
   end
 
+  # If your plugin creates notifications, and you'd like to consolidate/collapse similar ones,
+  # you're in the right place.
+  # This method receives a plan object, which must be an instance of `Notifications::ConsolidateNotifications`.
+  #
+  # Instead of using `Notification#create!`, you should use `Notification#consolidate_or_save!`,
+  # which will automatically pick your plan and apply it, updating an already consolidated notification,
+  # consolidating multiple ones, or creating a regular one.
+  #
+  # The rule object is quite complex. We strongly recommend you write tests to ensure your plugin consolidates notifications correctly.
+  #
+  # - Plan's documentation: https://github.com/discourse/discourse/blob/main/app/services/notifications/consolidate_notifications.rb
+  # - Base plans: https://github.com/discourse/discourse/blob/main/app/services/notifications/consolidation_planner.rb
+  def register_notification_consolidation_plan(plan)
+    raise ArgumentError.new("Not a consolidation plan") if plan.class != Notifications::ConsolidateNotifications
+    DiscoursePluginRegistry.register_notification_consolidation_plan(plan, self)
+  end
+
   protected
 
   def self.js_path