FIX: Migration paths were being forgotten

According to the [Rails
Source](https://github.com/rails/rails/blob/master/activerecord/lib/active_record/railties/databases.rake#L20)
the `ActiveRecord::Migrator.migrations_paths` are overwritten with the
value of `ActiveRecord::Tasks::DatabaseTasks.migrations_paths` every
time the config is loaded.

This caused a bug for Discourse development where if you ran:

`rake db:drop db:create db:migrate` in one line, you would not get our
post migrations, as those had a custom value for `migrations_paths`.

The fix is to use `ActiveRecord::Tasks::DatabaseTasks.migrations_paths`
to set up all our custom paths. Everything seems to work as expected.

config/initializers/000-post_migration.rb

@@ -1,7 +1,7 @@
 # frozen_string_literal: true
 
 unless Discourse.skip_post_deployment_migrations?
-  ActiveRecord::Migrator.migrations_paths << Rails.root.join(
+  ActiveRecord::Tasks::DatabaseTasks.migrations_paths << Rails.root.join(
     Discourse::DB_POST_MIGRATE_PATH
   ).to_s
 end

lib/plugin/instance.rb

@@ -526,7 +526,7 @@ class Plugin::Instance
     Rake.add_rakelib(File.dirname(path) + "/lib/tasks")
 
     # Automatically include migrations
-    migration_paths = ActiveRecord::Migrator.migrations_paths
+    migration_paths = ActiveRecord::Tasks::DatabaseTasks.migrations_paths
     migration_paths << File.dirname(path) + "/db/migrate"
 
     unless Discourse.skip_post_deployment_migrations?

lib/turbo_tests/runner.rb

@@ -84,7 +84,7 @@ module TurboTests
           .configurations["test"]
           .merge("database" => "discourse_test_1")
 
-      ActiveRecord::Migrator.migrations_paths = ['db/migrate', 'db/post_migrate']
+      ActiveRecord::Tasks::DatabaseTasks.migrations_paths = ['db/migrate', 'db/post_migrate']
 
       conn = ActiveRecord::Base.establish_connection(config).connection
       begin