Check if GitHub rate limite API was reached before update Webhooks

What does this MR do?

Checks if the job needs to sleep, and wait for the rate limit to be reseted before update each Webhook.

Are there points in the code the reviewer needs to double check?

No.

Why was this MR needed?

The import process can fail if the API rate limit was reached during the import process.

What are the relevant issue numbers?

https://gitlab.com/gitlab-org/gitlab-ce/issues/17498

Screenshots (if relevant)

Not relevant.