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/select-kit/templates/components/category-row.hbs

@@ -9,8 +9,8 @@
   </div>
 
   {{#if shouldDisplayDescription}}
-    <div class="category-desc">{{{dir-span description}}}</div>
+    <div class="category-desc">{{dir-span description}}</div>
   {{/if}}
 {{else}}
-  {{{label}}}
+  {{html-safe label}}
 {{/if}}

app/assets/javascripts/select-kit/templates/components/dropdown-select-box/dropdown-select-box-row.hbs

@@ -8,6 +8,6 @@
 {{/if}}
 
 <div class="texts">
-  <span class="name">{{{label}}}</span>
-  <span class="desc">{{{description}}}</span>
+  <span class="name">{{html-safe label}}</span>
+  <span class="desc">{{html-safe description}}</span>
 </div>

app/assets/javascripts/select-kit/templates/components/pinned-button.hbs

@@ -1,5 +1,5 @@
 {{pinned-options value=pinned topic=topic}}
 
 <p class="reason">
-  {{{reasonText}}}
+  {{html-safe reasonText}}
 </p>

app/assets/javascripts/select-kit/templates/components/topic-notifications-button.hbs

@@ -10,6 +10,6 @@
 
 {{#if appendReason}}
   <p class="reason">
-    {{{topic.details.notificationReasonText}}}
+    {{html-safe topic.details.notificationReasonText}}
   </p>
 {{/if}}