Accessibility: Set tabindex=0 on tab panels with no content

[emteknetnz]

Follow on from #1966

https://www.w3.org/WAI/ARIA/apg/patterns/tabs/ Keyboard Interaction Notes 4 states

When the tabpanel does not contain any focusable elements or the first element with content is not focusable, the tabpanel should set tabindex="0" to include it in the tab sequence of the page

An example of non-tabbable data might be a LiteralField that has <p>lorem ipusm</p> in it

We have 3x types of applicable tab panels

• The primary tabs (e.g. Content/Settings/History) that pjaxes in heaps of content
• Secondary tabs (e.g. getCMSFIelds()) that shows/hides content based using jquery
• React based tabs (<TabNav>) that do their own thing

Acceptance criteria

• Tab panels without tabbable content have tabindex="0" added to them

Kitchen sink CI

PRs

• NEW Add test classes for accessibility test silverstripe-frameworktest#248
• ENH Set tabindex=0 on tab panels with no focusable elements #2035
• DOC Changelog entry for non-interactive tab content panels developer-docs#832