crate.index: Load downloadsContext.version_downloads explicitly with a task

This fixes the following deprecations:
 - The get method on ember-data's PromiseManyArray is deprecated.

Author: eth3lbert

app/controllers/crate/version.js

@@ -24,7 +24,7 @@ export default class CrateVersionController extends Controller {
     this.stackedGraph = false;
   }
 
-  @alias('downloadsContext.version_downloads.content') downloads;
+  @alias('loadDownloadsTask.last.value') downloads;
   @alias('model.crate') crate;
   @alias('model.requestedVersion') requestedVersion;
   @alias('model.version') currentVersion;
@@ -62,4 +62,10 @@ export default class CrateVersionController extends Controller {
 
     return readme;
   });
+
+  // This task would be `perform()` in setupController
+  loadDownloadsTask = task(async () => {
+    let downloads = await this.downloadsContext.version_downloads;
+    return downloads;
+  });
 }

app/routes/crate/version.js

@@ -51,6 +51,9 @@ export default class VersionRoute extends Route {
     waitForPromise(controller.loadReadmeTask.perform()).catch(() => {
       // ignored
     });
+    waitForPromise(controller.loadDownloadsTask.perform()).catch(() => {
+      // ignored
+    });
 
     let { crate, version } = model;
     if (!crate.documentation || crate.documentation.startsWith('https://docs.rs/')) {