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
Also, yours only checks if it CONTAINS these extensions, not that they end with them. I'd use `end_with?`