DEV: Standardize ignored_columns removal comments (#9771)

app/models/category.rb

@@ -5,6 +5,7 @@ class Category < ActiveRecord::Base
     'none'
   ]
 
+  # TODO(2020-11-18): remove
   self.ignored_columns = %w{
     suppress_from_latest
   }

app/models/group.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 class Group < ActiveRecord::Base
+  # TODO(2021-04-22): remove
   self.ignored_columns = %w{
     automatic_membership_retroactive
   }

app/models/post.rb

@@ -10,7 +10,7 @@ class Post < ActiveRecord::Base
   include HasCustomFields
   include LimitedEdit
 
-  # remove line Jan 2021
+  # TODO(2021-01-04): remove
   self.ignored_columns = ["avg_time"]
 
   cattr_accessor :plugin_permitted_create_params

app/models/post_reply.rb

@@ -1,6 +1,7 @@
 # frozen_string_literal: true
 
 class PostReply < ActiveRecord::Base
+  # TODO(2020-01-17): remove
   self.ignored_columns = %w{
     reply_id
   }

app/models/topic.rb

@@ -10,7 +10,7 @@ class Topic < ActiveRecord::Base
   include LimitedEdit
   extend Forwardable
 
-  # remove line Jan 2021
+  # TODO(2021-01-04): remove
   self.ignored_columns = ["avg_time"]
 
   def_delegator :featured_users, :user_ids, :featured_user_ids

app/models/user_email.rb

@@ -1,6 +1,9 @@
 # frozen_string_literal: true
 
 class UserEmail < ActiveRecord::Base
+  # TODO(2020-04-24): remove
+  self.ignored_columns = ['canonical_email']
+
   belongs_to :user
 
   attr_accessor :skip_validate_email
@@ -17,8 +20,6 @@ class UserEmail < ActiveRecord::Base
 
   scope :secondary, -> { where(primary: false) }
 
-  self.ignored_columns = ['canonical_email']
-
   private
 
   def strip_downcase_email