Move the getCurrentAlertBanners method to a service

[andybroomfield]

This is the method that is currently inside the block class, but if we wanted to call this elsewhere it would be handy to have this as a separate service that can be called, with the block altered to call the service.

It should also be possible to

• Filter banners by type (currently this is supported by fetching block config)
• Include additional banners that aren't yet published (For supporting better banner previews)
• Exclude certain banners