Use ember-improved-cp for computed properties

# Problem

Computed properties are good, but the implicit two-way binding is _bad_. Stefan just introduced `ember-improved-cp`, which makes CP's read-only.

https://github.com/stefanpenner/ember-improved-cp
## Subtasks
- [ ] Identify all places where we could use this (maybe `git grep computed`?)
- [ ] Submit a pull implementing the changes
- [ ] Verify that nothing has broken

先运行 `git grep computed`，找出所有 computed property 的使用位置，然后查看关联的 `ember-improved-cp` 项目，并确定哪些位置适合进行此次更改。实现已确定的更新，提交 pull request，并验证没有任何内容被破坏。

由索引模型根据 Issue 内容生成。