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.

AMOS BEGIN
 MOV [event_all_submissions_downloaded, mod_assign],[eventallsubmissionsdownloaded, mod_assign]
 MOV [event_assessable_submitted, mod_assign],[eventassessablesubmitted, mod_assign]
 MOV [event_extension_granted, mod_assign],[eventextensiongranted, mod_assign]
 MOV [event_identities_revealed, mod_assign],[eventidentitiesrevealed, mod_assign]
 MOV [event_marker_updated, mod_assign],[eventmarkerupdated, mod_assign]
 MOV [event_statement_accepted, mod_assign],[eventstatementaccepted, mod_assign]
 MOV [event_submission_duplicated, mod_assign],[eventsubmissionduplicated, mod_assign]
 MOV [event_submission_graded, mod_assign],[eventsubmissiongraded, mod_assign]
 MOV [event_submission_locked, mod_assign],[eventsubmissionlocked, mod_assign]
 MOV [event_submission_status_updated, mod_assign],[eventsubmissionstatusupdated, mod_assign]
 MOV [event_submission_unlocked, mod_assign],[eventsubmissionunlocked, mod_assign]
 MOV [event_workflow_state_updated, mod_assign],[eventworkflowstateupdated, mod_assign]
AMOS END

Fix various incorrect urls
Fix incorrect phpdocs and other minor code style issues
Fix missing/incorrect strings

AMOS BEGIN
 MOV [eventblogassociationcreated, core_blog],[eventblogassociationadded, core_blog]
AMOS END

While restoring event context might not be present (deleted)
So before using context api's check context or use event properties