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.

app/assets/javascripts/discourse-common/helpers/html-safe.js.es6

@@ -0,0 +1,6 @@
+import { registerUnbound } from "discourse-common/lib/helpers";
+import { htmlSafe } from "@ember/template";
+
+registerUnbound("html-safe", function(string) {
+  return htmlSafe(string);
+});