Support clickable links in the Notifications class

Currently, only plain text can be shown by calling `Notifications.of(context).push('message')`. We'd want to support clickable links as well for places where we'd want to link to an external site for more guidance.

Start by locating the Notifications class and the existing Notifications.of(context).push('message') entry point, then inspect its call sites and related tests if present. Done means notifications can display clickable links to external guidance while preserving plain-text messages.

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