Add Age column to Report list pages

[nublarsec]

Closes #89.

This adds an Age column to the 3 current Report list pages, just like kubectl get vulnerabilityreport shows:

• Vulnerability Reports
• Config Audit Reports
• CIS Kube Bench Reports

It also has the absolute date as a tooltip on each Age value:

A couple of side notes, @danielpacak perhaps you can weigh in:

1. I wanted to use formatDuration() and LocaleDate from the Lens codebase to format datetimes. However they are not exported in the extension-specific NPM module, @k8slens/extensions, so I can't access them in the Starboard extension (as far as I can tell).
2. Instead of using those, I brought over formatDuration() in a new folder called utils/.
3. I also don't have access to UserStore from the Lens codebase, which means I can't see what the user's local timezone is for converting the absolute date. Instead I've just left it as UTC.

I'm somewhat new to Node/React/Electron—is there some other trick to being able to use these? Or are they not exported for a reason?