a11y addon should highlight violations tab by default

[domyen]

Describe the bug
a11y add-on should highlight “violations” tab when it loads. It currently shows the content for this tab but does not highlight the tab until you click on it.

To Reproduce
Steps to reproduce the behavior:

1. Go to a11y addon
2. See that the violations content is shown
3. But the violations tab is not highlighted (in blue)

Expected behavior
The violations tab should be highlighted

Screenshots
Bad

Good

[shilman]

Boo-yah!! I just released https://github.com/storybooks/storybook/releases/tag/v5.0.0-rc.2 containing PR #5646 that references this issue. Upgrade today to try it out!

Because it's a pre-release you can find it on the @next NPM tag.

Closing this issue. Please re-open if you think there's still more to do.