Should not allow teachers to remove the last teacher from class

lib/Class.js

@@ -244,6 +244,11 @@ class Class {
 		if (!validTypes.includes(u.type))
 			throw new Error('Invalid user type');
 
+		this.teachers = await this.getTeachers();
+
+		if (u.type === 'account' && this.teachers.length < 2)
+			throw new Error('Can\'t remove last teacher');
+
 		const sql = `
 			delete from ${u.type}ClassLink
 			where