✨ <amp-next-page> v2 default and templated separator elements

[wassgha]

Screenshot

Changes

• Implements an improved default UI for the separator pill (delimiting the end of an article and the start of the next) as designed here.
• Implements templating for custom separator elements which provides the title, url and image of the next article. Example:

<div separator class="amp-next-page-default-separator">
     <template type="amp-mustache">
          <div class="separator-content">
            <amp-img src={{image}} layout="fixed" height="16" width="16"></amp-img>
            <span>Next article: {{title}}</span>
          </div>
    </template>
</div>

Todo

• Add unit tests

[wassgha]

/cc @alanorozco @caroqliu this one should be ready now :)