FEATURE: adds a new plugin api to decorate plugin outlets (#8937)

```
api.decoratePluginOutlet(
  "discovery-list-container-top",
  elem => {
    if (elem.classList.contains("foo")) {
      elem.style.backgroundColor = "yellow";
    }
  }
);
```

test/javascripts/acceptance/plugin-outlet-decorator-test.js.es6

@@ -0,0 +1,61 @@
+import { acceptance } from "helpers/qunit-helpers";
+import { withPluginApi } from "discourse/lib/plugin-api";
+
+const PREFIX = "javascripts/single-test/connectors";
+acceptance("Plugin Outlet - Decorator", {
+  loggedIn: true,
+
+  beforeEach() {
+    Ember.TEMPLATES[
+      `${PREFIX}/discovery-list-container-top/foo`
+    ] = Ember.HTMLBars.compile("FOO");
+    Ember.TEMPLATES[
+      `${PREFIX}/discovery-list-container-top/bar`
+    ] = Ember.HTMLBars.compile("BAR");
+
+    withPluginApi("0.8.38", api => {
+      api.decoratePluginOutlet(
+        "discovery-list-container-top",
+        (elem, args) => {
+          if (elem.classList.contains("foo")) {
+            elem.style.backgroundColor = "yellow";
+
+            if (args.category) {
+              elem.classList.add("in-category");
+            } else {
+              elem.classList.remove("in-category");
+            }
+          }
+        },
+        { id: "yellow-decorator" }
+      );
+    });
+  },
+
+  afterEach() {
+    delete Ember.TEMPLATES[`${PREFIX}/discovery-list-container-top/foo`];
+    delete Ember.TEMPLATES[`${PREFIX}/discovery-list-container-top/bar`];
+  }
+});
+
+QUnit.test(
+  "Calls the plugin callback with the rendered outlet",
+  async assert => {
+    await visit("/");
+
+    const fooConnector = find(".discovery-list-container-top-outlet.foo ")[0];
+    const barConnector = find(".discovery-list-container-top-outlet.bar ")[0];
+
+    assert.ok(exists(fooConnector));
+    assert.equal(fooConnector.style.backgroundColor, "yellow");
+    assert.equal(barConnector.style.backgroundColor, "");
+
+    await visit("/c/bug");
+
+    assert.ok(fooConnector.classList.contains("in-category"));
+
+    await visit("/");
+
+    assert.notOk(fooConnector.classList.contains("in-category"));
+  }
+);

test/javascripts/helpers/qunit-helpers.js.es6

@@ -18,6 +18,7 @@ import { initSearchData } from "discourse/widgets/search-menu";
 import { resetDecorators } from "discourse/widgets/widget";
 import { resetWidgetCleanCallbacks } from "discourse/components/mount-widget";
 import { resetDecorators as resetPostCookedDecorators } from "discourse/widgets/post-cooked";
+import { resetDecorators as resetPluginOutletDecorators } from "discourse/components/plugin-connector";
 import { resetCache as resetOneboxCache } from "pretty-text/oneboxer";
 import { resetCustomPostMessageCallbacks } from "discourse/controllers/topic";
 import User from "discourse/models/user";
@@ -128,6 +129,7 @@ export function acceptance(name, options) {
       initSearchData();
       resetDecorators();
       resetPostCookedDecorators();
+      resetPluginOutletDecorators();
       resetOneboxCache();
       resetCustomPostMessageCallbacks();
       Discourse._runInitializer("instanceInitializers", function(