left sidebar should default to dashboard, not metrics def

[hamilton]

• clicking on a metrics thing on the left sidebar should go to the dashboard if the dashboard is valid
• if it isn't valid, it should go to the metrics def
• if it is empty, it should go to the metrics def
• we should call the section "DASHBOARDS" + use the little dashboard icon

[hamilton]

@AdityaHegde could you take al ook at this? I tried to get it in but the metrics def didn't quite seem to have the right information in the MetricsDefinitionAssets.svelte component. The measures and dimensions seemed to be empty even if they weren't actually empty.