Super sidebar focus

Description

Raised in Slack: https://gitlab.slack.com/archives/C04G34U119T/p1678374282211139

Currently the super sidebar has tabindex="-1" as it is given focus when the "Expand sidebar" button is pressed. This adds the user-agent default focus-ring styles to the sidebar <aside> element. It appears the sidebar is also given focus when the browser window is resized.

Outcome

• Update sidebar focus-ring styles to be consistent with other interactive elements (@include gl-focus)
• Investigate how/when sidebar is given focus and limit visual focus indication to the result of user action i.e. pressing "Expand sidebar" button only.