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).

test/unit/post_event_test.rb

@@ -1,20 +0,0 @@
-require 'test_helper'
-
-class PostEventTest < ActiveSupport::TestCase
-  def setup
-    @user = create(:user, created_at: 2.weeks.ago)
-    @post = create(:post)
-    @post_flag = create(:post_flag, creator: @user, post: @post)
-    @post.update(is_deleted: true)
-    @post_appeal = create(:post_appeal, creator: @user, post: @post)
-  end
-
-  context "PostEvent.find_for_post" do
-    should "work" do
-      results = PostEvent.find_for_post(@post.id)
-      assert_equal(2, results.size)
-      assert_equal("appeal", results[0].type_name)
-      assert_equal("flag", results[1].type_name)
-    end
-  end
-end