Remove fields, options and blocks on uninstall

Problem/Motivation

It would be nice if when this module is uninstalled, even if sitewide_alert is still installed, if there were no remnants of the items it added.

Acceptance Criteria

Upon uninstalling uswds alert

1. style settings added to the form during install are removed
2. fields added to the sitewide_alert entity are removed

Proposed resolution

Remaining tasks

Steps to reproduce