[Page attachments] [Outlink] Anchor link for bot rendering.

The CTA is now an anchor element so the `remote-content` element does not need to be:
https://github.com/ampproject/amphtml/blob/2f6158b665b180194eb0b85e151319e2b4eaad8d/extensions/amp-story/1.0/amp-story-page-attachment.js#L210-L214

This was originally done for bot rendering when the CTA was not an anchor element.

When converting this to a div we will need to confirm that bots will be able to crawl the anchor link on the CTA button.

Currently the CTA button is not rendered in vertical mode.
We might need to render the outlink CTAs to be sure they are crawled by bots.

Instructions to set up a custom device in chrome devtools for debugging bot rendering:

User agent string:
`Mozilla/5.0 (compatible; Googlebot/2.1; +http://www.google.com/bot.html)`

@ampproject/wg-stories