Widgetize Severity sidebar component

In Incident sidebar (issue boards and incident page), the Severity component should be a widget.

Architecture docs for reference: https://docs.gitlab.com/ee/development/fe_guide/widgets.html#widgets

Availability and Testing

Job e2e:package-and-test will need to be run in the MR(s) that introduces the change.

Ensure test coverage in unit/integration tests.