Fix the master show link preview setting and add back the per room setting

Title

BUG: 486126

src/models/messagecontentmodel.h

@@ -15,7 +15,7 @@
 #include "neochatroom.h"
 
 struct MessageComponent {
-    MessageComponentType::Type type;
+    MessageComponentType::Type type = MessageComponentType::Other;
     QString content;
     QVariantMap attributes;
 
@@ -23,6 +23,11 @@ struct MessageComponent {
     {
         return type == right.type && content == right.content && attributes == right.attributes;
     }
+
+    bool isEmpty() const
+    {
+        return type == MessageComponentType::Other;
+    }
 };
 
 /**