fix(a11y): add explicit labels to alert icons

[billhimmelsbach]

Often information is conveyed by the icons of our alerts (this is an error or warning, is this an external link) that is not conveyed explicitly by the text. These icons are then not presentational, and should be labeled.

Changes

• Explicitly adds aria-labels to svgs in alerts, field level alerts, and alert links
• removes alt text in favor of aria-label since alt text is not universally supported on svg elements (changes to the Icon element's use of alt text will be in a follow-up PR)

How to test this PR

1. Are the svgs now properly labeled on alerts, field level alerts, and alert links?

Screenshots

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