network graph: relayout when the slider is released, not on every pixel

layoutGraph() runs 140 iterations of an O(n^2) force loop. Measured by calling
it standalone with the same constants:

     20 nodes     24 ms
     50 nodes     57 ms
    100 nodes    199 ms
    200 nodes    729 ms   (NODE_LIMIT)

The edge length slider called it from oninput, which a range input fires dozens
of times per drag, each call blocking the main thread. Dragging it on a level 2
graph froze the tab for several seconds.

oninput now only moves the label, which is what has to stay live, and onchange
does the layout once, on release.

webui-src/app/network/network_graph.js

@@ -209,10 +209,15 @@ const NetworkGraph = () => {
           `Edge length ${edgeLength}`,
           m('input[type=range][min=60][max=180][step=5]', {
             value: edgeLength,
+            //  The label follows the slider, the layout waits for the release:
+            //  layoutGraph() is 140 iterations of an O(n^2) force loop, which
+            //  measures 24 ms at 20 nodes, 199 ms at 100 and 729 ms at the 200
+            //  node cap. A range input fires oninput dozens of times per drag,
+            //  each one blocking the main thread for that long.
             oninput: (event) => {
               edgeLength = Number(event.target.value);
-              redrawLayout();
             },
+            onchange: redrawLayout,
           }),
         ]),
         m('.network-graph__zoom-control', [