MDL-41101 mod_assign: removed underscores from event string keys

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
MDL-41101 mod_assign: introduced event base class

Also performed the following -

1) Edited existing events to use the new base class.
2) Renamed event_test.php to events_test.php to match the rest of core.
3) Moved the event tests from locallib_test.php to events_test.php.
MDL-41101 mod_assign: replaced 'view all' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view submission' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view feedback' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view grading form' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view submission grading table' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view submit assignment form' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view' add_to_log call for the reveal identities confirmation page with an event
MDL-41101 mod_assign: replaced 'submission statement accepted' add_to_log call with an event

Also removed unnecessary add_to_log call.
MDL-41101 mod_assign: replaced 'view confirm submit assignment form' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view' add_to_log call for the submission status page with an event
MDL-41101 mod_assign: replaced 'view batch set marking workflow state' add_to_log call with an event
MDL-41101 mod_assign: replaced 'view batch set marker allocation' add_to_log call with an event
MDL-41101 mod_assign: replaced 'grade submission' add_to_log call with an event
MDL-41101 mod_assign: replaced 'save grading feedback' add_to_log call with an event