improve mixpanel usage statistics (#2238)

Other changes:
- Always display first start dialog if privacy options are unset (e.g. if the user closed GPT4All without selecting them)
- LocalDocs scanQueue is now always deferred
- Fix a potential crash in magic_match
- LocalDocs indexing is now started after the first start dialog is dismissed so usage stats are included

Signed-off-by: Jared Van Bortel <jared@nomic.ai>

gpt4all-chat/llm.cpp

@@ -49,7 +49,7 @@ bool LLM::checkForUpdates() const
     #pragma message "offline installer build will not check for updates!"
     return QDesktopServices::openUrl(QUrl("https://gpt4all.io/"));
     #else
-    Network::globalInstance()->sendCheckForUpdates();
+    Network::globalInstance()->trackEvent("check_for_updates");
 
 #if defined(Q_OS_LINUX)
     QString tool("maintenancetool");