FIX: Add checkbox-label class to site setting checkboxes (#24075)

Followup to e2d9117378, which
made these labels bold because they were missing the correct
class.

app/assets/javascripts/admin/addon/components/site-settings/bool.hbs

@@ -1,4 +1,4 @@
-<label>
+<label class="checkbox-label">
   <Input @type="checkbox" @checked={{this.enabled}} />
   <span>{{html-safe this.setting.description}}</span>
 </label>