Introduce ember-changeset and ember-changeset-validations

# Problem

Too much complexity is wrapped up in `save` right now when dealing with models.

Where possible, we should work with changesets, using [`ember-changeset`](https://github.com/DockYard/ember-changeset), which will also allow us to use validations.
## Subtasks
- [ ] Identify places where we can introduce changesets
- [ ] Break out separate issues for tackling these

先找出目前複雜度集中的模型儲存流程，接著確認哪些位置可以使用 changesets 和驗證。當這些候選區域已完成文件記錄，並且已針對每項具體變更建立個別的 issue 後，這項工作就完成了。

由索引模型根據 Issue 內容生成。