DEV: Clean up Ember imports (#8979)

Includes:
* Import `computed` helpers
* Import `@ember/application`
* Import `isBlank` from `@ember/utils`
* Import `A` from `@ember/array`
* Import `EmberArray` from `@ember/array`
* Import `ArrayProxy` from `@ember/array/proxy`
* Import `warn` from `@ember/debug`
* Import `EmberObject` from `@ember/object`
* Import `Application` from `@ember/application`
* Import `EmberRouter` from `@ember/routing/router`
* Import `isPresent` from `@ember/utils`
* Import `computed` from `@ember/object`
* Import `guidFor` from `@ember/object`
* Import `isArray` from `@ember/array`
* Import `TextField` from `@ember/component`
* Import `TextArea` from `@ember/component`
* Import `Promise` from `rsvp`
* Import `Evented` from `@ember/object/evented`
* Replace deprecated `ember-addons/ember-computed-decorators` imports

plugins/poll/assets/javascripts/initializers/extend-for-poll.js.es6

@@ -1,3 +1,4 @@
+import EmberObject from "@ember/object";
 import { withPluginApi } from "discourse/lib/plugin-api";
 import { observes } from "discourse-common/utils/decorators";
 import { getRegister } from "discourse-common/lib/get-owner";
@@ -44,7 +45,7 @@ function initializePolls(api) {
           if (existing) {
             this._polls[p.name].setProperties(p);
           } else {
-            this._polls[p.name] = Ember.Object.create(p);
+            this._polls[p.name] = EmberObject.create(p);
           }
         });
         this.set("pollsObject", this._polls);
@@ -78,8 +79,8 @@ function initializePolls(api) {
       if (quotedId) {
         const quotedPost = post.quoted[quotedId];
         if (quotedPost) {
-          post = Ember.Object.create(quotedPost);
-          poll = Ember.Object.create(
+          post = EmberObject.create(quotedPost);
+          poll = EmberObject.create(
             quotedPost.polls.find(p => p.name === pollName)
           );
           vote = quotedPost.polls_votes || {};