Subscribe to writable folders only

app/src/main/java/eu/faircode/email/AdapterFolder.java

@@ -451,7 +451,7 @@ public class AdapterFolder extends RecyclerView.Adapter<AdapterFolder.ViewHolder
 
                 if (folder.account != null && folder.accountProtocol == EntityAccount.TYPE_IMAP) {
                     boolean subscriptions = prefs.getBoolean("subscriptions", false);
-                    if (subscriptions)
+                    if (subscriptions && !folder.read_only)
                         popupMenu.getMenu().add(Menu.NONE, R.string.title_subscribe, 9, R.string.title_subscribe)
                                 .setCheckable(true).setChecked(folder.subscribed != null && folder.subscribed);