MDL-41101 mod_assign: introduced event base class

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.
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
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