draw better dashboard images

Scaling big plots to small thumbnails results in bad images that barely
show any details.
We solve this by calling gnuplot a second time to generate the
thumbnails. They don't have any labels and are rendered in the required
size, so that not scaling is necessary.
Thumbnails have to be requested explicitly, because it can be expensive
to compute them.

pdb-ui/src/main/java/org/lucares/pdbui/PdbController.java

@@ -130,7 +130,7 @@ public class PdbController implements HardcodedValues, PropertyKeys {
 
 				final String thumbnailUrl = result.getThumbnailPath() != null
 						? WEB_IMAGE_OUTPUT_PATH + "/" + result.getThumbnailName()
-						: imageUrl;
+						: "img/no-thumbnail.png";
 
 				final PlotResponseStats stats = PlotResponseStats.fromDataSeries(result.getDataSeries());
 				final PlotResponse plotResponse = new PlotResponse(stats, imageUrl, thumbnailUrl);
@@ -184,6 +184,7 @@ public class PdbController implements HardcodedValues, PropertyKeys {
 			plotSettings.setPlotType(plotType);
 			plotSettings.setAggregate(PlotSettingsTransformer.toAggregateInternal(aggregate));
 			plotSettings.setKeyOutside(keyOutside);
+			plotSettings.setGenerateThumbnail(false);
 
 			if (plotterLock.tryLock()) {
 				try {