Remove duplicate XHR request when requesting new pipeline page

When a user clicked on another page in the pipeline page, the following would happen:

1. PipelinesService.getPipelines() would be called to fetch the new page
2. In the success handler, Poll.restart() would be called
3. This would invoke Poll.makeRequest(), which would fire off a new request.

To fix this, we introduce a enable(), which will:

1. Update the request data accordingly
2. Clear the old timeout if necessary and start a new timer

Closes https://gitlab.com/gitlab-org/gitlab-ce/issues/58095