Return module icon URL in calendar WebServices

In MDL-75571, some changes were made to display the custom LTI icon in the Timeline and Upcoming events blocks. In order to do the same in the app without adding new WebServices calls, we'd need the calendar WebServices to return the icon URL of each event (if they're activity events).

In the app we also display the module icon inside calendar events, so the WebServices that should be modified to return the icon URL are:

core_calendar_get_action_events_by_timesort

core_calendar_get_action_events_by_course

core_calendar_get_action_events_by_courses

core_calendar_get_calendar_event_by_id

core_calendar_get_calendar_day_view

core_calendar_get_calendar_monthly_view

core_calendar_get_calendar_upcoming_view