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.

See merge request !4509