feat: Labels reordering

Closes #289

client/src/models/Board.js

@@ -175,6 +175,10 @@ export default class extends BaseModel {
     return this.memberships.orderBy('id');
   }
 
+  getOrderedLabelsQuerySet() {
+    return this.labels.orderBy('position');
+  }
+
   getOrderedListsQuerySet() {
     return this.lists.orderBy('position');
   }

client/src/models/Label.js

@@ -8,6 +8,7 @@ export default class extends BaseModel {
 
   static fields = {
     id: attr(),
+    position: attr(),
     name: attr(),
     color: attr(),
     boardId: fk({