register missing push devices at login (#3792)

save the push token of new device even if push notifications are not
enabled and provide a way to register the push device at login

unregister device if there already is a push token saved unless the
new token has already been registered.

also the `unregister_push_device` function used the wrong argument
cf. 08d380900b/src/Core/Services/Implementations/RelayPushRegistrationService.cs (L43)

src/api/admin.rs

@@ -413,7 +413,7 @@ async fn deauth_user(uuid: &str, _token: AdminToken, mut conn: DbConn, nt: Notif
 
     if CONFIG.push_enabled() {
         for device in Device::find_push_devices_by_user(&user.uuid, &mut conn).await {
-            match unregister_push_device(device.uuid).await {
+            match unregister_push_device(device.push_uuid).await {
                 Ok(r) => r,
                 Err(e) => error!("Unable to unregister devices from Bitwarden server: {}", e),
             };