A11y improvements for file upload

[dsamojlenko]

What this does

Improves a11y of the File Uploader:

• Makes the upload input element visible to screen readers (hidden >> sr-only)
• Adds visual focus to the upload button (using focus-within on parent div)
• Makes the delete links visible to screen readers (span >> a)
• Sets focus back to the upload button after deleting a file

TODO

• Need to run this by Julianna
• Need to roll this out to the Supporting Documents component which has similar issues

[CalvinRodo]

Code looks fine just missing the following features:

• Upload button is missing hover styles.
• Upload button doesn't show up in tab order.
• Upload button doesn't get highlighting when it's selected.

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