SidebarNavigation: use the bell icon

app/icons/alert.svg

@@ -1 +0,0 @@
-<svg viewBox="0 0 24 24" stroke="currentColor" fill="none" xmlns="http://www.w3.org/2000/svg"><path d="M19 14v6a2 2 0 0 1-2 2H4a2 2 0 0 1-2-2V7a2 2 0 0 1 2-2h6M19 9a4 4 0 1 0 0-8 4 4 0 0 0 0 8Z" stroke-width="2" stroke-linecap="round" stroke-linejoin="round"/></svg>

app/soapbox/components/sidebar-navigation.tsx

@@ -157,7 +157,7 @@ const SidebarNavigation = () => {
 
             <SidebarNavigationLink
               to='/notifications'
-              icon={require('icons/alert.svg')}
+              icon={require('@tabler/icons/icons/bell.svg')}
               count={notificationCount}
               text={<FormattedMessage id='tabs_bar.notifications' defaultMessage='Notifications' />}
             />