Reschedule checking of invalid links with HTTP status 429/529

[timobrembeck]

When links are invalid due to a server's rate limiting (usually denoted with HTTP status 429) or a temporary overload (unofficial standard seems to be 529), I think it would make sense to not update the (last) status of the link and keep last_checked unchanged. This way, the link can easier be checked again in the future even when EXTERNAL_RECHECK_INTERVAL has not yet passed.
Optionally, we could also re-schedule these tasks automatically as part of cron.py.

[claudep]

+1 to keep last_checked unchanged in this case.