Add `dismissible` option to `Alert` component

Created by: lrhacker

To help ensure we adhere to a standardized design pattern for dismissible alerts, it'd be better to have dismissible as part of the Alert component API (rather than relying on consumers to pick their own button/icon/text to use for dismissal). It may look something like:

<Alert dismissible={true} onDismiss={() => {}}>
    Something you can dismiss
</Alert>

Final design for the dismissible Alert UI is still TBD. See this Slack thread for more context.