Accessibility fix

[oadetoye]

We had accessibility issues with JAWS not seeing toasty notifications. This fix adds the ARIA alert role to each toasty and also removes the container from the DOM if there are no toasties. The latter is an attempt to fix an issue where JAWS reads the alerts twice.

[CLAassistant]

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1 out of 2 committers have signed the CLA.

✅ Ehesp
❌ oadetoye
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