FEATURE: New site setting, whitelisted_link_domains

If provided, users who normally couldn't post links (say, due to a
low trust level), can post links to those specific hosts.

app/assets/javascripts/discourse/components/composer-editor.js.es6

@@ -47,6 +47,11 @@ export default Ember.Component.extend({
     }
   },
 
+  @computed
+  showLink() {
+    return this.currentUser && this.currentUser.get('link_posting_access') !== 'none';
+  },
+
   @computed('composer.requiredCategoryMissing', 'composer.replyLength')
   disableTextarea(requiredCategoryMissing, replyLength) {
     return requiredCategoryMissing && replyLength === 0;

app/assets/javascripts/discourse/templates/components/composer-editor.hbs

@@ -11,7 +11,7 @@
   validation=validation
   loading=composer.loading
   forcePreview=forcePreview
-  showLink=currentUser.can_post_link
+  showLink=showLink
   composerEvents=true
   onExpandPopupMenuOptions="onExpandPopupMenuOptions"
   onPopupMenuAction=onPopupMenuAction