Remove :perform_http_request after_create callback

- Remove the perform_http_request() method in WebpageRequest as well as
  the corresponding after_create callback and run
  WebpageService.perform_http_request() directly during the
  WebpageRequestsController#create action.
- In order to accommodate the removal of the after_create callback in
  WebpageRequest, add a perform_http_request transient attribute to the
  webpage_requests factory that defaults to false. When set to true,
  this attribute runs WebpageService.perform_http_request() for the
  newly created WebpageRequest instance. This is used to accurately test
  the behavior of the controllers.