Safe Exam Browser support reform:
- capture both RequestHash and ConfigKeyHash from the request headers and use both to validate the request. This enables to choose to just enforce the plain conf, instead of the whole conf+binaries combination, which is platform-specific
- support storing multiple keys, required e.g. to support multiple platform-specific keys
- make suppling of the config file optional
- extend automated tests to cover new features
- clanup redundant index
- return config file using its stored filename