webhooks: skip webhooks with expired credentials
The documentation states [1]:
> When the original authentication method becomes invalid (such as the
> expiration of or revocation of an OAuth 2.0 bearer token), the
> webhook is disabled.
However, this is currently not the case. Expired webhooks are indeed
filtered out in virtually all GraphQL queries (by means of core-go's
FilterWebhooks [2]), so users cannot see or delete them. They are _not_
filtered out upon scheduling, however. This commit fixes that.
The symptoms of this are that active webhooks may simply not be run - if
a user has both an expired and a valid, active webhook for some event,
the scheduling will retrieve both, fail on the expired one, and stop
processing, without any feedback to the user who scheduled the hooks.
This is a problem across all services, so core-go seems like the best
place to fix this.
[1]: https://man.sr.ht/graphql.md#webhook-authentication
[2]: https://git.sr.ht/~sircmpwn/core-go/tree/master/item/webhooks/config.go#L74,81