DEV: enforces ember-template-lint: no-triple-curlies (#9150)

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.

plugins/discourse-local-dates/assets/javascripts/discourse/templates/components/discourse-local-dates-create-form.hbs

@@ -85,7 +85,7 @@
             <label class="control-label">
               {{i18n "discourse_local_dates.create.form.recurring_title"}}
             </label>
-            <p>{{{i18n "discourse_local_dates.create.form.recurring_description"}}}</p>
+            <p>{{i18n "discourse_local_dates.create.form.recurring_description"}}</p>
             <div class="controls">
               {{combo-box
                 content=recurringOptions