Move currentDiscussionId into Vuex store

Summary

app/assets/javascripts/notes/components/notes_app.vue is storing currentDiscussionId. This should be moved to the store so that it remains in sync with other discussion navigation buttons.

Improvements

Store currentDiscussionId in vuex store. More info here: https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30144#note_200432180

Risks

Discussion navigation could break. Our existing unit tests should catch this, but will need to be augmented by modifying the state outside of the current next/previous discussion calls to ensure that navigation still works when currentDiscussionId updated outside of their respective components.

i.e. notes_app.vue uses keyboard shortcuts to navigate. So test that those still work after updating state.

Involved components

app/assets/javascripts/notes/components/notes_app.vue app/assets/javascripts/notes/stores