FIX: Rails 6 multisite migrations and plugin migrations

Prior to this change plugin migrations were not working and multisite
migrations not working.

Rails internals changed so we need to account for it.

Specifically semantics of `db:migrate` in rails changed so it is sort of
a "multisite:migrate".

lib/plugin/instance.rb

@@ -516,7 +516,7 @@ class Plugin::Instance
     Rake.add_rakelib(File.dirname(path) + "/lib/tasks")
 
     # Automatically include migrations
-    migration_paths = Rails.configuration.paths["db/migrate"]
+    migration_paths = ActiveRecord::Migrator.migrations_paths
     migration_paths << File.dirname(path) + "/db/migrate"
 
     unless Discourse.skip_post_deployment_migrations?