[amp story page attachment] CTA is not tab-able

[processprocess]

Description

Inline attachment CTAs are currently not tab-able.
This breaks keyboard a11y.

A couple approaches to making it visible to the a11y dom tree:

1. Adding an empty href attribute.
2. Changing it to a button instead of an anchor element.

Reproduction Steps

1. Open a story with an attachment such as examples/amp-story/attachment.html on the dev server.
2. press tab to cycle through tab-able elements.