Add two methods to the axios_utils Jest mock:

- `waitFor(url)`, which returns a Promise that resolves when the
  next request to `url` finishes.
- `waitForAll()`, which returns a Promise that resolves when all
  pending requests finish.

Also adds comment to explain why this works on the diff-tab
when it's only used in `notes-app`.

https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/30144#note_204066538

Add keyboard shortcuts `p` and `n` to navigate duscussions.

- applied functional component patch
- fixed border radius for list
- fixed Karma test to take a correct li amount

- all commits squashed to make danger review happy

Events listeners have been fixed to ensure UI interactions are
properly handled in discussion notes