DEV: enforces ember-template-lint: no-triple-curlies (#9165)

This pr replaces `{{{ }}}` usage by a {{html-safe}} helper. While it doesn't solve the underlying issue, it gives us a path forward without risking breaking too much existing behavior.

Also introduces an htmlSafe computed macro:

```
import { htmlSafe } from "discourse/lib/computed";

htmlDescription: htmlSafe("description")
```

Overtime {{html-safe}} usage should be removed and moved to components properties or specialized components/helpers.

test/javascripts/components/html-safe-helper-test.js.es6

@@ -0,0 +1,14 @@
+import componentTest from "helpers/component-test";
+moduleForComponent("html-safe-helper", { integration: true });
+
+componentTest("default", {
+  template: "{{html-safe string}}",
+
+  beforeEach() {
+    this.set("string", "<p class='cookies'>biscuits</p>");
+  },
+
+  async test(assert) {
+    assert.ok(exists("p.cookies"), "it displays the string as html");
+  }
+});

test/javascripts/lib/computed-test.js.es6

@@ -5,7 +5,8 @@ import {
   propertyNotEqual,
   fmt,
   i18n,
-  url
+  url,
+  htmlSafe
 } from "discourse/lib/computed";
 
 QUnit.module("lib:computed", {
@@ -154,3 +155,12 @@ QUnit.test("url", assert => {
     "it supports urls with a prefix"
   );
 });
+
+QUnit.test("htmlSafe", assert => {
+  const cookies = "<p>cookies and <b>biscuits</b></p>";
+  const t = EmberObject.extend({
+    desc: htmlSafe("cookies")
+  }).create({ cookies });
+
+  assert.equal(t.get("desc").string, cookies);
+});