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 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