improve the "Show hash" interactivity

[kyranjamie]

The show hash toggle functionality could be improved.

Some suggestions:

• Increase the click target. Right now, only the 16x16px arrow box is clickable. The whole row should be clickable.
• Make accessible. It's a div and should be a button. It can't be accessed with keyboard navigation.
• Interactive states Hover/focus states are missing.

cc/ @landitus @beguene

[markmhendrickson]

@kyranjamie can we close this as completed and released?

[kyranjamie]

These changes are in the unreleased PR