Create email actions

- Test email content:
  https://www.google.com/webmasters/markup-tester/u/0/
- Email sender validation:
  https://developers.google.com/gmail/markup/registering-with-google
- Actions doccumentation:
  https://developers.google.com/gmail/markup/overview

taiga/base/templates/emails/base-body-html.jinja

@@ -447,5 +447,6 @@
                 </tr>
             </table>
         </center>
+        {% block extra %}{% endblock %}
     </body>
 </html>

taiga/base/templates/emails/hero-body-html.jinja

@@ -421,5 +421,6 @@
                 </tr>
             </table>
         </center>
+        {% block extra %}{% endblock %}
     </body>
 </html>

taiga/base/templates/emails/updates-body-html.jinja

@@ -483,5 +483,6 @@
                 </tr>
             </table>
         </center>
+        {% block extra %}{% endblock %}
     </body>
 </html>

taiga/projects/notifications/templates/emails/issues/issue-create-body-html.jinja

@@ -9,3 +9,13 @@
     <p><small>The Taiga Team</small></p>
     {% endtrans %}
 {% endblock %}
+
+{% block extra %}
+<div itemscope itemtype="http://schema.org/EmailMessage">
+    <div itemprop="action" itemscope itemtype="http://schema.org/ViewAction">
+        <link itemprop="url" href="{{ resolve_front_url("issue", project.slug, snapshot.ref) }}"></link>
+        <meta itemprop="name" content="{{ _("See issue #{ref}").format(ref=snapshot.ref) }}"></meta>
+    </div>
+    <meta itemprop="description" content="{{ _("See issue #{ref} '{subject}' detail page").format(ref=snapshot.ref, subject=snapshot.subject) }}"></meta>
+</div>
+{% endblock %}