Improve design of alerts page

[struan]

This is the first PR in a series to update the look of the alerts page. It only updates the display of existing alerts and not the creation etc. It splits alerts into keyword alerts and representative alerts, although x is mentioned alerts still count as keyword alerts for now.

It doesn't really touch any of the back end other to add some extra properties to alert objects.

It's also missing any translations for the new text.

I'm not proposing this is merged, but is used as the first part of several which will include changes to the alert creation UX.

Part of #1824