Accessibility: Media: Improve accessibility of the status and error messages in the Image Editor.

- improves focus management by moving focus to the notices, if any, or to the first "tabbable" element
- this avoids a focus loss and helps Braille-only and screen magnification users to be aware of the messages
- adds an ARIA role `alert` to all the notices 
- uses `wp.a11y.speak()` to announce messages to assistive technology
- this way, all visual users will see the messages while assistive technology users will get an audible message
- uses `wp.i18n` for translatable strings in `wp-admin/js/image-edit.js`

Props anevins, ryanshoover, antpb, SergeyBiryukov, afercia.
See #20491.
Fixes #47147.

Built from https://develop.svn.wordpress.org/trunk@48375


git-svn-id: https://core.svn.wordpress.org/trunk@48144 1a063a9b-81f0-0310-95a4-ce76da25c4cd