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/controllers/post_approvals_controller.rb

@@ -15,4 +15,12 @@ class PostApprovalsController < ApplicationController
 
     respond_with(@post_approvals)
   end
+
+  def show
+    @approval = authorize PostApproval.find(params[:id])
+
+    respond_with(@approval) do |format|
+      format.html { redirect_to post_approvals_path(search: { id: @approval.id }) }
+    end
+  end
 end