Validate image URLs with custom validator

Previously: image model only validates for presence of URL
problem: garbage links add broken images to posts, and even blank URLs show up on the page
solution: 
1. create an ImageUrlValidator that tests for properly formatted URLs that point to the right kind of resource (.png, .jp(e)g, .gif)
2. force reload of images from database so that non-persisted urls (the ones that don't pass validation) don't show up as broken image links
You can just do: `url.end_with?(".png", ".jpg", ".jpeg", "gif")`. No regex needed!