FR: Dashboard empty state

[paolodamico]

General description

Extends from #1694. This issue intends to implement the dashboard welcome and graph empty states (design here). When there's not enough events to show a graph in the default dashboard, the following behavior should occur:

• The main graph images are faded (tooltips still enabled but with sample data).
• A description (that communicates that this is a preview of how the actual graph will look like) of the graph is added.
• The sample graphs (the ones that need actual user input to work), show a large "configure" button on hover.
• If there are empty graphs shown (not sample graphs) on the default dashboard we add a general notice at the top of the page.

Additional context

Related to #1659.

Thank you for your feature request – we love each and every one!