Merge developer meetings into other meetings on the events page

On the events page, https://mariadb.org/events/, I suggest merging "Developer meetings" and other meetings. Some Server Fests are in the one list, some in the other. So let's do away with "Upcoming developer meetings" and "Past developer meetings", and put the contents from "Upcoming developer meetings" into "Upcoming events" and "Past developer meetings" into "Past events and conferences" in the correct year and date order.