posts: rework post events page.

* Add a global /post_events page that shows the history of all approvals,
  disapprovals, flags, appeals, and replacements on a single page.

* Redesign the /posts/:id/events page to show all approval, disapproval,
  flag, appeal, and replacement events for a single post (before it only
  showed approvals, flags, and appeals).

* Remove the replacement history link from the post show page. Replacements
  are now included in the post events page (closes #4948: Highlighed replacements).

* Add /post_approvals/:id and /post_replacements/:id routes (these are
  used by the "Details" link on the post events page).

app/policies/post_event_policy.rb

@@ -0,0 +1,36 @@
+# frozen_string_literal: true
+
+class PostEventPolicy < ApplicationPolicy
+  def can_see_creator?
+    case event.model_type
+    when "PostFlag"
+      policy(event.model).can_view_flagger?
+    when "PostDisapproval"
+      policy(event.model).can_view_creator?
+    else
+      true
+    end
+  end
+
+  def api_attributes
+    [:model_type, :model_id, :post_id, (:creator_id if can_see_creator?), :event_at].compact
+  end
+
+  def visible_for_search(events, attribute)
+    case attribute
+    in :creator | :creator_id
+      events.model_types.map do |type|
+        attr = attribute
+        attr = attr.to_s.gsub("creator", "uploader").to_sym if type == "Post"
+        attr = attr.to_s.gsub("creator", "user").to_sym if type in "PostApproval" | "PostDisapproval"
+
+        # XXX ordering by created_at desc is a query planner hack to make Postgres use the right indexes.
+        events.where(model: type.constantize.visible_for_search(attr, user).order(created_at: :desc))
+      end.reduce(:or)
+    else
+      events
+    end
+  end
+
+  alias_method :event, :record
+end