Make notes vuex store namespaced
The following discussion from !34432 should be addressed:
- [ ] @mrincon started a [discussion](https://gitlab.com/gitlab-org/gitlab/-/merge_requests/34432#note_380334300):
> **Suggestion (for follow up):** It took me a while to understand why this didn't work:
>
> ```js
> ...mapState('notes', ['selectedCommentPosition', 'selectedCommentPositionHover']),
> ```
>
> It turns out it's because the `notes` module is not namespaced (`namespaced: true`).
>
> Is there a reason for this? I see some other instances where you had to use getters or other "tricks" to map to it. On the other hand `diffs` is namespaced.
>
> Is this something we could follow up? I think it's the reason why some of these extra getters have been added to: `app/assets/javascripts/notes/stores/getters.js`, I think it could help clean up the code.
issue