Allow amp-iframe in amp-story-page-attachment

## Describe the new feature or change to an existing feature you'd like to see

amp-iframe appears to function as expected within an amp-story-page-attachment, but it doesn't pass validation.

An example use case would be to embed google maps in an attachment.

[validator-amp-story.protoascii#L708](https://github.com/ampproject/amphtml/blob/master/extensions/amp-story/validator-amp-story.protoascii#L708) mentions whitelisting as many components as possible, unless it results in bad UX or security issues. Do amp-iframes fall under either of those categories?

## Describe alternatives you've considered

For the map use case, you could probably generate map images and use amp-img but the experience is not the same as supporting a full blown map embed.