Accessibility: No screen reader feedback when uploading or saving files/folders

[GuySartorelli]

When uploading files, creating folders, and saving files/folders (and maybe other actions as well) there's no feedback provided to screen readers.

For example a screen reader user has no indication that an upload has completed, let alone whether it was successful or not.

When saving a file or folder, there is an indication that something has happened, but not that the save was successful.

There may be other admin sections likewise affected.

Acceptance criteria

• Toast messages are used to indicate whether actions in asset admin were successful or not
• Appropriate aria attributes are used to indicate failed actions (e.g. consider aria-invalid and aria-errormessage
• Actions in other CMS sections are evaluated to check if they are appropriately accessible - if not, raise a separate card to fix them