Change "recent" scopes to sort by "id"

These scopes can just sort by the "id" column in descending order to
achieve the same result. An added benefit is being able to perform a
backwards index scan (depending on the rest of the final query) instead
of having to actually sort data.

app/models/concerns/issuable.rb

@@ -24,7 +24,7 @@ module Issuable
 
     scope :authored, ->(user) { where(author_id: user) }
     scope :assigned_to, ->(u) { where(assignee_id: u.id)}
-    scope :recent, -> { order("created_at DESC") }
+    scope :recent, -> { reorder(id: :desc) }
     scope :assigned, -> { where("assignee_id IS NOT NULL") }
     scope :unassigned, -> { where("assignee_id IS NULL") }
     scope :of_projects, ->(ids) { where(project_id: ids) }

app/models/event.rb

@@ -45,7 +45,7 @@ class Event < ActiveRecord::Base
   after_create :reset_project_activity
 
   # Scopes
-  scope :recent, -> { order(created_at: :desc) }
+  scope :recent, -> { reorder(id: :desc) }
   scope :code_push, -> { where(action: PUSHED) }
   scope :in_projects, ->(project_ids) { where(project_id: project_ids).recent }
   scope :with_associations, -> { includes(project: :namespace) }