refactor all static config related code

• replace all document_category, link_category, event_type, news_type related configuration and handling on settings
  to use readable names and automatically generate categories or types if they are missing
• remove all methods and constants in SchoolPeer and related modules that use these settings
• add new class SchoolSiteConfig to handle all these new methods that can be called from different places
	like the detail widget for settings
	or filter modules to handle permissions
• rename getClassLink() or getServiceLink() to getLink() because the prefix is unnecessary
• update migration file to stress the importance of the procedures in order not to brake any relationships

IMPORTANT: make sure you read the notes in data/migration_20_steps.md before you instanciate any related detail widget