Add ability to submit a site

@wallawe Here we go. So this is a bit cludgy in the UX right now. You submit a site and we use the `screencap` gem to fetch the image then store the File in `tmp` and upload it to S3. We can add height/width options on `screencap` so the image sizes are consistent so let's chat about that. Of course we could also scrap this and use something like embedly or some other service.