Add shortcut for "Add reaction" and make "Add reaction" navigable from keyboard using arrow keys

Description

Problem: it's not possible to add a reaction without using a pointing device.

image

Proposal

  1. Create a shortcut for adding a reaction to the latest comment. Slack already uses Ctrl+Shift+\ for this so we could consider using the same.
  2. Make "Add reaction" window navigable using arrow keys, just like in Slack.

Links / references

Documentation blurb

Overview

  • What is it?
    • Productivity improvement for keyboard-only use of GitLab.
  • Why should someone use this feature?
    • To save time and wrists.
  • What is the underlying (business) problem?
    • User productivity.
  • How do you use this feature?
    • Hit Ctrl+Shift+\ and use arrows.

Use cases

You propose an improvement in GitLab in https://gitlab.com/gitlab-org/gitlab-ce/issues/26596#note_35779807. @sarahod analyses the problem and confirms it would be a great addition. I love her answer and want to express it with an emoji reaction.

Feature checklist

Make sure these are completed before closing the issue, with a link to the relevant commit.