Image for Twitter Card auto-tweets is too large

[jcn]

This is not strictly a bug in our code, but since we use our Atom feed to serve twitter images (via zapier), sometimes this doesn't work because we are just feeding the raw image url, which can sometimes be huge. We should detect when this image is too large for the feed (something reasonable, though twitter cards can be a max of 4096x4096) and downsize if needed.

We should not upsize smaller photos, however.