MDL-37101 webservices:Implement ws api core_calendar_create_calendar_events

Conflicts:

	calendar/externallib.php
MDL-37101 phpunit: phpunit tests for ws api core_calendar_create_calendar_events
MDL-37101 donot use donot.

Conflicts:

	calendar/externallib.php
MDL-37533 Assign: assign_print_overview does not correctly display assignments that need grading

Conflicts:

	mod/assign/lib.php
MDL-37533 Assign: Use the constant for the submitted status in the query (and parameterize the query)

Conflicts:

	mod/assign/lib.php
MDL-37533 Assign: Fix grading filters to show only assignments needing grading