feat: Add tasks reordering

Closes #50, closes #232

client/src/models/Card.js

@@ -206,7 +206,7 @@ export default class extends Model {
   }
 
   getOrderedTasksQuerySet() {
-    return this.tasks.orderBy('id');
+    return this.tasks.orderBy('position');
   }
 
   getOrderedAttachmentsQuerySet() {

client/src/models/Task.js

@@ -7,6 +7,7 @@ export default class extends Model {
 
   static fields = {
     id: attr(),
+    position: attr(),
     name: attr(),
     isCompleted: attr({
       getDefault: () => false,