Implemented school management system

- CRUD operations on school table
- Based on URL information (RESTful)
  -> ID passed by URL is not checked for edit and delete operations: this
     should be done in the future
- Added a configurer for properties loading purpose
- Modified AdminController and ErrorController
  -> Paths are now static final strings instead of inline strings
- Created SchoolService for SchoolController
- Problem found: bad character encoding on create and edit operations
- Created properties file for messages (still empty and not used)
- Created an "AuthenticationSuccessHandler" to custom some operations
  when users authenticate with success:
  -> "onAuthentificationSuccess" to add more session variables