DEV: Promote old post-deploy migrations to pre-deploy migrations (#13477)

Having a large number of post-deploy migrations running out-of-numerical-sequence with pre-deploy migrations can be problematic. For example, if we have the sequence

- db/migrate/2017... - add column
- db/post_migrate/2018... - drop the column
- db/migrate/2021... - add the same column again

It will work fine in numerical order. But if you run the pre-deploy migrations **followed by** the post-deploy migrations, you will not get the same result.

Our post-deploy system is designed to allow for seamless upgrades of Discourse. However, it is reasonable for us to only support this totally seamless experience for a limited period of time. This commit moves all post_deploy migrations which are more than 1 year old (i.e. more than 2 major Discourse versions ago) into the regular pre-deploy migrations directory. This limits the impact of any edge cases caused by out-of-numerical-sequence migrations.

db/migrate/20190312194528_drop_email_user_options_columns.rb

@@ -0,0 +1,23 @@
+# frozen_string_literal: true
+
+require 'migration/column_dropper'
+
+class DropEmailUserOptionsColumns < ActiveRecord::Migration[5.2]
+  DROPPED_COLUMNS ||= {
+    user_options: %i{
+        email_direct
+        email_private_messages
+        email_always
+      },
+  }
+
+  def up
+    DROPPED_COLUMNS.each do |table, columns|
+      Migration::ColumnDropper.execute_drop(table, columns)
+    end
+  end
+
+  def down
+    raise ActiveRecord::IrreversibleMigration
+  end
+end