Create "Playground" tab for transient dashboards

[juliusv]

Sometimes people want to use PromDash to just explore data and create transient graphs. Currently they need to create a full dashboard to do that. There should be a "Playground" tab in PromDash which lets people build transient Prometheus graphs, but with the ability of sending them around as links.

[grobie]

For a playground to test expressions, it'd make it even easier to have an always visible, big expression field. In the end what we have in the prometheus server explorer. Just better 💃