Add emailsignup atom

[amyhughes]

This should support an atom type for Editorial email sign up boxes, currently included as html snippets like this:

<figure class="element element-embed" data-alt="Business email signup">
<iframe src="https://www.theguardian.com/email/form/plaintone/3887" height="42px" data-form-title="Sign up for Business Today" data-form-description="Get the headlines and editors' picks every weekday morning." scrolling="no" seamless="" frameborder="0" class="iframed--overflow-hidden email-sub__iframe js-email-sub__iframe js-email-sub__iframe--article" data-form-success-desc="Thanks for signing up" data-node-uid="2"></iframe>
<figcaption class="caption caption--img">
Sign up to the daily Business Today email or follow Guardian Business on Twitter at @BusinessDesk
</figcaption>
</figure>

This is rendered like below:

An atom type for these will allow editors to create reusable components for different email lists, reducing the chances that an email sign up box will be added signing users up to the wrong email (copying, pasting and editing the html is error prone!).

As an aside, the new atom should be rendered more like this:

TODO:

• Confirm which fields are actually necessary (this is a guess)
• Add field to Elasticsearch mapping in CAPI

[amyhughes]

Don't need this - list name or list id