DEV: Fix function prototype deprecations (#8681)

* DEV: Fix the function prototype observers deprecation

DEPRECATION: Function prototype extensions have been deprecated, please migrate from function(){}.observes('foo') to observer('foo', function() {}). [deprecation id: function-prototype-extensions.observes] See https://deprecations.emberjs.com/v3.x/#toc_function-prototype-extensions-observes for more details.

* DEV: Fix the function prototype event listeners deprecation

DEPRECATION: Function prototype extensions have been deprecated, please migrate from function(){}.on('foo') to on('foo', function() {}). [deprecation id: function-prototype-extensions.on] See https://deprecations.emberjs.com/v3.x/#toc_function-prototype-extensions-on for more details.

* DEV: Simplify `default as` imports

Co-authored-by: Joffrey JAFFEUX <j.jaffeux@gmail.com>

app/assets/javascripts/admin/models/color-scheme-color.js.es6

@@ -1,5 +1,4 @@
-import {
-  default as discourseComputed,
+import discourseComputed, {
   observes,
   on
 } from "discourse-common/utils/decorators";

app/assets/javascripts/admin/models/theme.js.es6

@@ -2,7 +2,7 @@ import { get } from "@ember/object";
 import { isEmpty } from "@ember/utils";
 import { or, gt } from "@ember/object/computed";
 import RestModel from "discourse/models/rest";
-import { default as discourseComputed } from "discourse-common/utils/decorators";
+import discourseComputed from "discourse-common/utils/decorators";
 import { popupAjaxError } from "discourse/lib/ajax-error";
 import { ajax } from "discourse/lib/ajax";
 import { escapeExpression } from "discourse/lib/utilities";

app/assets/javascripts/admin/models/web-hook.js.es6

@@ -2,10 +2,7 @@ import { isEmpty } from "@ember/utils";
 import RestModel from "discourse/models/rest";
 import Category from "discourse/models/category";
 import Group from "discourse/models/group";
-import {
-  default as discourseComputed,
-  observes
-} from "discourse-common/utils/decorators";
+import discourseComputed, { observes } from "discourse-common/utils/decorators";
 import Site from "discourse/models/site";
 
 export default RestModel.extend({