Make chart panels focusable via keyboard

What does this MR do?

Adds the tabindex property to the chart panels inside the monitoring dashboard. This allows for keyboard navigation, subsequent features will add keyboard shortcuts (beyond the tab key)

Issue: #212476 (closed)

Screenshots

Navigating via the keyboard
Screen_Recording_2020-04-14_at_16.00.15

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides