Refactor logging

[pi-sigma]

The current implementation of logging has a few problems:

• The login_sucess message is contained in error_messages on the BaseBackend, which is confusing
• The message strings are translated, even though they are only (?) used for logging, which means they end up translated in log aggregating systems
• The logging calls are wrapped in a way that makes it impossible to pass arguments to the extra paremeter of the logger calls. This is important because we want to pass arguments to extra containing information about the context of the particular logger call