Sensor page: less confusion between statistics & graph

[nhoening]

As a user, the sensor page's graph and statistic table are confusing. They might diverge in what time range they show, as well as other things like which data sources.

This issue proposes some changes to the statistics table, to relieve confusion. It only does work that is straightforward. More improvements like horizon selection are out of scope.

• Statistics default to the time range selected for the graph. (now the default is for all time). When all time is selected & loaded, display a warning (bootstrap alert) below the statistics: "This table is showing all-time statistics - the graph still shows the selected time range."
• The statistics table shows one column per data source. Right now, data is already in the browser for all sources (the function _get_sensor_stats() already groups selected data by source), so we would not show the toggle, but rather all sources. Order left-to-right by number of entries, and limit the visible size of the statistics table - add a scrollbar if there are many sources.
• Just like the graph (when it is not in playback), statistics per default show only most recent beliefs. That should also be explained in a help icon. This one might make the query slower. We should maybe do test in conjunction with the materialized-view PR.