Make a dashboard its widgets: drop the pages and sections nobody drew

frontend/tests/widgets.spec.ts

@@ -89,7 +89,7 @@ test.beforeAll(async ({ browser }) => {
   // Two rows taller than the default panel: the tiles below already fill it,
   // and a widget past the last row is clipped rather than drawn.
   dashboard.canvas_height = 1400
-  dashboard.pages[0].sections[0].widgets = [
+  dashboard.widgets = [
     {
       id: "load",
       type: "bar",
@@ -202,7 +202,7 @@ test.beforeAll(async ({ browser }) => {
   const stack = await (
     await api(page, `/dashboards/${stackName}`, { method: "POST" })
   ).json()
-  stack.pages[0].sections[0].widgets = [
+  stack.widgets = [
     {
       id: "split",
       type: "bar",