Add default sort by severity

[duncanmmacleod]

Is your feature request related to a problem? Please describe.

For large cube groups the default service listing shown when clicking on a cube is sorted by 'name', meaning the problem services that the cube highlighted aren't visible by default (in general).

Describe the solution you'd like

The default service list shown when clicking on a cube should be sorted by 'severity', so that those services who directly influenced the cube summary display are top of the list.

Describe alternatives you've considered

Additional context

The cube view is great, this would save me a few extra clicks every time I use the cubes.