Restore the possibility of requesting reruns of public datasets!

Switching from the original list of available datasets to the lightweight data portal has been great, but we forgot one thing: we've lost the ability to request datasets' reruns! This is easily fixed by adding this possibility to the data portal. There are two places where an "On Demand" button could be added:

• in the list view, when the user has unrolled a dataset, add a last entry below all the variants for this dataset "> [On Demand data]"
• in the map view, once the user has clicked on a marker and a popup opens, add a "[On Demand data]" on the same line as the "Station: xxx" but right justified

Both buttons send the user to the on demand configuration page that we had.