Refactor WebpageScreenshot model and specs

- Reduce calls to generate_screenshot during testing
- Change temporary location for screenshots to tmp/screenshots instead
  of tmp/
- Create screenshot after creating a WebpageResponse as opposed to
  within the get_url method
- Don't upload a new screenshot to S3 if it already exists
- Run PhantomJS in testing environment again instead of creating
  temporary file
- Disable callbacks in WebpageResponse and WebpageScreenshot during
  testing
- Update WebpageRequests controller VCR cassettes