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/post.rb

@@ -70,6 +70,7 @@ class Post < ApplicationRecord
   has_many :favorites, dependent: :destroy
   has_many :replacements, class_name: "PostReplacement", :dependent => :destroy
   has_many :ai_tags, through: :media_asset
+  has_many :events, class_name: "PostEvent"
 
   attr_accessor :old_tag_string, :old_parent_id, :old_source, :old_rating, :has_constraints, :disable_versioning, :post_edit
 
@@ -1631,7 +1632,7 @@ class Post < ApplicationRecord
   def self.available_includes
     # attributes accessible through the ?only= parameter
     %i[
-      uploader approver flags appeals parent children notes
+      uploader approver flags appeals events parent children notes
       comments approvals disapprovals replacements pixiv_ugoira_frame_data
       artist_commentary media_asset ai_tags
     ]

app/models/post_event.rb

@@ -1,82 +1,40 @@
 # frozen_string_literal: true
 
-class PostEvent
-  include ActiveModel::Model
-  include ActiveModel::Serializers::JSON
-  include ActiveModel::Serializers::Xml
+class PostEvent < ApplicationRecord
+  belongs_to :model, polymorphic: true
+  belongs_to :creator, class_name: "User"
+  belongs_to :post
 
-  attr_accessor :event
-
-  delegate :created_at, to: :event
-
-  def self.find_for_post(post_id)
-    post = Post.find(post_id)
-    (post.appeals + post.flags + post.approvals).sort_by(&:created_at).reverse.map { |e| new(event: e) }
+  def self.model_types
+    %w[Post PostAppeal PostApproval PostDisapproval PostFlag PostReplacement]
   end
 
-  def type_name
-    case event
-    when PostFlag
-      "flag"
-    when PostAppeal
-      "appeal"
-    when PostApproval
-      "approval"
-    end
+  def self.visible(user)
+    all
   end
 
-  def type
-    type_name.first
-  end
+  def self.search(params, current_user)
+    q = search_attributes(params, [:model, :post, :creator, :event_at], current_user: current_user)
 
-  def reason
-    event.try(:reason) || ""
-  end
-
-  def creator_id
-    event.try(:creator_id) || event.try(:user_id)
-  end
-
-  def creator
-    event.try(:creator) || event.try(:user)
-  end
-
-  def status
-    if event.is_a?(PostApproval)
-      "approved"
-    elsif (event.is_a?(PostAppeal) && event.succeeded?) || (event.is_a?(PostFlag) && event.rejected?)
-      "approved"
-    elsif (event.is_a?(PostAppeal) && event.rejected?) || (event.is_a?(PostFlag) && event.succeeded?)
-      "deleted"
+    case params[:order]
+    when "event_at_asc"
+      q = q.order(event_at: :asc, model_id: :asc)
     else
-      "pending"
+      q = q.apply_default_order(params)
     end
+
+    q
   end
 
-  def is_creator_visible?(user = CurrentUser.user)
-    case event
-    when PostAppeal, PostApproval
-      true
-    when PostFlag
-      flag = event
-      Pundit.policy!(user, flag).can_view_flagger?
-    end
+  def self.default_order
+    order(event_at: :desc, model_id: :desc)
   end
 
-  def attributes
-    {
-      creator_id: nil,
-      created_at: nil,
-      reason: nil,
-      status: nil,
-      type: nil,
-    }
+  def self.available_includes
+    [:post, :model] # XXX creator isn't included because it leaks flagger/disapprover names
   end
 
-  # XXX can't use hidden_attributes because we don't inherit from ApplicationRecord.
-  def serializable_hash(options = {})
-    hash = super
-    hash = hash.except(:creator_id) unless is_creator_visible?
-    hash
+  def readonly?
+    true
   end
 end

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