Add reason param to POST /api/v1/accounts REST API (#12064)

For approval-required registrations mode

app/controllers/api/v1/accounts_controller.rb

@@ -78,7 +78,7 @@ class Api::V1::AccountsController < Api::BaseController
   end
 
   def account_params
-    params.permit(:username, :email, :password, :agreement, :locale)
+    params.permit(:username, :email, :password, :agreement, :locale, :reason)
   end
 
   def check_enabled_registrations