Improve the handling of switching link preivews on and off.

First make sure that the global setting is tied into the room setting, previously it was a bit of a patchwork that worked more by luck than judgement. The two levels of global and room level are properly tied together in a hierarchy.

Add a message in the room when global notifcations re turned off. This has caused confusion in the past when people don't realise there are 2 levels.

src/neochatconnection.cpp

@@ -145,6 +145,10 @@ void NeoChatConnection::connectSignals()
     connect(NeoChatConfig::self(), &NeoChatConfig::PreferUsingEncryptionChanged, this, [] {
         setDirectChatEncryptionDefault(NeoChatConfig::preferUsingEncryption());
     });
+    setGlobalUrlPreviewEnabled(NeoChatConfig::showLinkPreview());
+    connect(NeoChatConfig::self(), &NeoChatConfig::ShowLinkPreviewChanged, this, [this]() {
+        setGlobalUrlPreviewEnabled(NeoChatConfig::showLinkPreview());
+    });
 }
 
 int NeoChatConnection::badgeNotificationCount() const
@@ -167,6 +171,25 @@ void NeoChatConnection::refreshBadgeNotificationCount()
     }
 }
 
+bool NeoChatConnection::globalUrlPreviewEnabled()
+{
+    return m_globalUrlPreviewEnabled;
+}
+
+void NeoChatConnection::setGlobalUrlPreviewEnabled(bool newState)
+{
+    if (m_globalUrlPreviewEnabled == newState) {
+        return;
+    }
+
+    m_globalUrlPreviewEnabled = newState;
+    if (!m_globalUrlPreviewEnabled) {
+        m_linkPreviewers.clear();
+    }
+    NeoChatConfig::setShowLinkPreview(m_globalUrlPreviewEnabled);
+    Q_EMIT globalUrlPreviewEnabledChanged();
+}
+
 void NeoChatConnection::logout(bool serverSideLogout)
 {
     SettingsGroup(u"Accounts"_s).remove(userId());
@@ -505,7 +528,7 @@ QString NeoChatConnection::accountDataJsonString(const QString &type) const
 
 LinkPreviewer *NeoChatConnection::previewerForLink(const QUrl &link)
 {
-    if (!NeoChatConfig::showLinkPreview()) {
+    if (!m_globalUrlPreviewEnabled) {
         return nullptr;
     }