Remove trailing whitespace
Refactor and clean up WebpageScreenshot more

- Use a random prefix and the WebpageRequest slug for the
  WebpageScreenshot filename
- Reduce length of generate_screenshot and upload_screenshot methods by
  returning true at the beginning when the screenshot file or screenshot
  object exists as opposed to using a conditional
- Change double-quoted strings to single-quoted where string
  interpolation is unnecessary
- Remove screenshot images generated during tests before the database is
  reset