Show ftb show/hide tooltip only on hover

Rinku Crequested to merge
gitlab-community/gitlab-org/gitlab:rc_show_ftb_tooltip_only_on_hover into master

What does this MR do and why?

shows "Hide file tree browser" tooltip only on hover

References

#591277

Screenshots or screen recordings

Before After

How to set up and validate locally

  1. Checkout this branch and run gdk locally
  2. Enable the repository_file_tree_browser feature flag from http://localhost:3000/rails/features if not already activated
  3. Go to any project repository page Code > Repository
  4. Click the button to expand or collapse the file tree browser
  5. Notice the tooltip appearing only on hover
  6. Compare to master branch where it was shown on button focus as well

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