Make deleting a period harder

We have a danger zone with multiple warnings for deleting a whole camp. However, a period can contain almost as much information as a whole camp, and deleting a period currently is way easier. There have been cases where periods have been deleted accidentally (while trying to move it to a new date, by creating a second period, manually moving the activities and deleting the former period; the period moving feature was not discovered in this process).

We already have a validation in place which prevents deleting the last period of a camp, which might contribute to how rarely this has happened so far. We could add danger zone style warnings or checks for deleting periods too, e.g. the user has to enter the period name in order to delete it, or display more information about schedule entries already present in the period, etc.