Don't redirect from https to http when serving the GIF

[davidkazuhiro]

This causes mixed content issues and means LGTM GIFs don't get shown in some browsers if the site uses the block-all-mixed-content flag

[rahulsom]

Thanks @davidkazuhiro! I've made some changes that would help future images in b809782.

Earlier, we used to just capture the image url. With this change, if the image is not an https imgur url, it gets uploaded to imgur and then referenced, at submit time. That allows dependably using https.