No link to OEmbeded images

[markjaquith]

When we use OEmbed to insert an image, we just do an image tag. No link to the image page. This violates Flickr's terms of service (and might violate others). It also eliminates some potentially useful information that might be on that page. Comments, notes, etc. We should wrap the image in the URL provided.

[nacin]

(In [16444]) When inserting an image via oEmbed, wrap it in a link. props MarkJaquith, fixes #14945.