Validate all images are actually images during the tests

[MarshallOfSound]

We could do a HEAD request to all image URL's to ensure the mimeType returned is actually an image.

This would prevent things like #430 where the URL added didn't actually point to an image 👍

Existing tests: https://github.com/electron/electron-apps/blob/master/test/human-data.js#L100

[zeke]

Yeah I had a similar thought a while back: #88

We'd need to make sure this check only happens on newly-added (or newly updated) apps, so as to avoid making a jillion network requests. :)