Address several Checkstyle warnings

server/src/main/java/umm3601/todo/Todo.java

@@ -1,7 +1,18 @@
 package umm3601.todo;
 
+// Normally you'd want all fields to be private, but
+// we need the fields in this class to be public since
+// they will be written to by the Jackson library. We
+// need to suppress the Visibility Modifier
+// (https://checkstyle.sourceforge.io/config_design.html#VisibilityModifier)
+// check in CheckStyle so that we don't get a failed
+// build when Gradle runs CheckStyle.
+@SuppressWarnings({"VisibilityModifier"})
 public class Todo {
-  public String _id;
+  // By default Java field names shouldn't start with underscores.
+  // Here, though, we *have* to use the name `_id` to match the
+  // name of the field in the database.
+  @SuppressWarnings({"MemberName"})  public String _id;
   public String owner;
   public boolean status;
   public String body;

server/src/test/java/umm3601/todo/TodoControllerSpec.java

@@ -15,8 +15,6 @@
 
 import io.javalin.http.Context;
 import umm3601.Main;
-import umm3601.user.UserController;
-import umm3601.user.UserDatabase;
 
 public class TodoControllerSpec {
   private static TodoDatabase db;