* Replaces SEVERITIES const with Severity enum in BannerAlert
* Replaced SEVERITIES with Severity in banner-alert.constants.js
* Used BANNER_ALERT_SEVERITIES instead of SEVERITIES in banner-alert.stories.js
* Updated README for usage of Severity instead of SEVERITIES
* Updates to stories and docs link
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Co-authored-by: Brad Decker <bhdecker84@gmail.com>
Co-authored-by: georgewrmarshall <george.marshall@consensys.net>