Audit: handle redirected resources in byte efficiency audits

While working on #4456, I came across some strange behavior for our byte-efficiency audits on redirected images, two things to note.

* Byte-efficiency audits should handle savings on images whose initial request 301/302'd (image src doesn't change on the redirect, so we're subtracting the cost from the wrong request)
* Might want to audit about images/non-document resources that redirected at all? could be an extension of our main document redirect audit