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/models/user.rb

@@ -115,6 +115,7 @@ class User < ApplicationRecord
   has_many :post_appeals, foreign_key: :creator_id
   has_many :post_approvals, :dependent => :destroy
   has_many :post_disapprovals, :dependent => :destroy
+  has_many :post_events, class_name: "PostEvent", foreign_key: :creator_id
   has_many :post_flags, foreign_key: :creator_id
   has_many :post_votes
   has_many :post_versions, foreign_key: :updater_id