Provide an attribute to opt-in to the accordion within pf-notification-drawer

[benjaminapetersen]

The accordion is a nice feature when needed, but it would also be handy to be able to have a list of simple events. My use case may not need the rollup. This could be an attribute, or simply based on the structure of the notificationGroups object passed. If it is a simple array, skip the pf-fixed-accordion.

[benjaminapetersen]

If not an opt in to accordion, it could work to still use the headings, just not collapsible. That may be a good way to keep the data structure the same & just toggle the presentation.

[jeff-phillips-18]

Fixed vai #526