We need a way to decide whether an action can currently be completed
(e.g., an assignment where submissions are not yet open can not
be actioned, even though it may be displayed on the upcomming events
list).
To do this, the action interface now contains an is_actionable method.
Part of MDL-55611 epic.
Course modules can now implement the function mod_{module}_core_calendar_provide_event_action
which allows them to return an action to attach to an event.
Part of MDL-55611 epic.