Also performed the following -
1) Deprecated the assignment's add_to_log function (part of it's functionality exists in the base class to set the legacy log data).
2) Edited existing events to use the new base class.
3) Renamed event_test.php to events_test.php to match the rest of core.
4) Moved the event tests from locallib_test.php to events_test.php.
5) When setting legacy data use lang_string, rather than get_string, as we may not be using them if legacy log is disabled.
