Currently, the reviewables tab in the user menu shows pending reviewables at the top of the menu and fills the remaining space in the menu with old/handled reviewables. This PR makes the revieables tab show only pending reviewables and hides the tab altogether from the menu if there are no pending reviewables. We're going to follow-up with another change soon that will show pending reviewables in the main tab of the user menu.
Internal topic: t/73220.