WebPurifyImage#imgStatus: require exactly one of imgid or customimgid

Though it is undocumented, Webpurify's API can alternatively find and
return the status of an image by customimgid rather than imgid. This
patch allows us to query imgstatus in this way.

From Webpurify support:

> As for looking up the status using the custom id, this feature is in
> BETA, however you are more than welcome to use it. We plan on
> bringing it out of beta, which simply means posting the
> documentation on our website by the New Year.