Add holiday management functionality with CRUD operations

- Implement HolidaysController for managing holidays
- Create Holiday model with validation for recurrence types
- Add holiday form and views for creating, editing, and listing holidays
- Integrate holiday checks into customer appointment scheduling
- Update locale file for holiday-related labels and error messages
- Add migration for holidays table
- Update plugin version and add holidays to admin menu

config/routes.rb

@@ -10,6 +10,7 @@
 
 RedmineApp::Application.routes.draw do
 
+  resources :holidays, except: [:show] # Exclude :show since we don't need a separate page for individual holidays
   resources :customer_appointments do
     collection do
       get :customer_options