Add API endpoint for the dashboard data

[JVT038]

This PR adds an endpoint which returns all the data that's shown in the dashboard. Additionally, it also returns the dashboard rows and their properties (e.g. whether they're extended)

If the row is hidden, then the data for that row won't be included in the API response. The new Vue frontend will hide / collapse that row and if the user clicks on it, the client manually requests the data for the row. This adds lazy loading support for the dashboard row, (partially?) fixing #394

Part of #572