WebUI: Implement "Secure" flag for session cookie

Closes #11724.

Option is enabled by default for users using qBittorrent's built-in HTTPS capabilities. This flag will never be set if qBittorrent is using plain HTTP.

Users using HTTPS reverse proxies, like "qbt <-> (http) <-> proxy <-> (https) <-> user" should override the flag in the proxy in order to set it, if they wish to do so.

src/webui/webapplication.h

@@ -153,6 +153,7 @@ private:
     QStringList m_domainList;
     bool m_isClickjackingProtectionEnabled;
     bool m_isCSRFProtectionEnabled;
+    bool m_isSecureCookieEnabled;
     bool m_isHostHeaderValidationEnabled;
     bool m_isHttpsEnabled;
     QString m_contentSecurityPolicy;