mass update calendar -> calendar_(medium)
The calendar tag is meant for images that make up calendar pages, while calendar (object) is supposed to be for the actual object. However often people don't bother checking and just tag both cases with calendar. This update will prevent people from mistakingly using the wrong tag.
EDIT: The bulk update request #1750 (forum #148985) has been approved by @Hillside_Moose.
Updated by DanbooruBot
