FIX: We should include resumable.js in the admin bundle (#13049)

Normally we'd use `ember-auto-import` for this, but it's not run on
our admin tree due to the quirky way we load it conditionally.
Instead we'll append it at the bottom like our Rails app does.

app/assets/javascripts/discourse/ember-cli-build.js

@@ -36,6 +36,10 @@ module.exports = function (defaults) {
   app.import(vendorJs + "jquery.fileupload.js");
   app.import(vendorJs + "jquery.autoellipsis-1.0.10.js");
 
+  let adminVendor = funnel(vendorJs, {
+    files: ["resumable.js"],
+  });
+
   return mergeTrees([
     discourseScss(`${discourseRoot}/app/assets/stylesheets`, "testem.scss"),
     createI18nTree(discourseRoot, vendorJs),
@@ -46,7 +50,7 @@ module.exports = function (defaults) {
       destDir: "assets/highlightjs",
     }),
     digest(
-      concat(app.options.adminTree, {
+      concat(mergeTrees([app.options.adminTree, adminVendor]), {
         outputFile: `assets/admin.js`,
       })
     ),