Accessibility: Screen reader use is problematic in the Files modal

[sachajudd]

At the moment when the files modal is opened the focus is set on the whole dialog causing all of the information in the modal to be read out at once for a screen reader user.
It would be good to investigate if we could set the focus to the “Files” title when the modal is opened so a screen reader can tab through the appropriate fields themselves.

An alternative approach would be to add a label (either with aria-label or aria-labelled-by attributes) to the element that is already getting focus.

Notes

Check all ways the modal can be opened, e.g

• insert image in the WYSIWYG
• "choose existing" in UploadField
• others?

kitchen sink ci
test failures are existing

PRs

(have moved back to refinement as we need to work out screen reader testing and development approach first)

• ENH Add autoFocus prop to Breadcrumb silverstripe-admin#1998
• ENH Autofocus modal title #1609
• DOC File modal title automatically gets focus developer-docs#816

[sachajudd]

@clarkepaul could this be added to any future experience debt epics?