handlers: permisison check for all pages + retrieve user-specific permissions for pages (if any)

handlers/handlerpage.cpp

@@ -18,6 +18,7 @@ LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
 OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE
 SOFTWARE.
 */
+#include <optional>
 #include "handlerpage.h"
 
 Response HandlerPage::handle(const Request &r)