Add Schema.org view actions to change emails

Adds a GMail action to "View Change" to the footer of change emails.

Feature: Issue 3772
Change-Id: I233e90f29d7c7a1edc2d6e8343787a98a66a214b

gerrit-server/src/main/resources/com/google/gerrit/server/mail/ChangeFooterHtml.soy

@@ -46,4 +46,13 @@
     Gerrit-Branch: {$branch.shortName}<br/>
     Gerrit-Owner: {$change.ownerEmail}
   </p>
+
+  {if $email.changeUrl}
+    <div itemscope itemtype="http://schema.org/EmailMessage">
+      <div itemscope itemprop="action" itemtype="http://schema.org/ViewAction">
+        <link itemprop="url" href="{$email.changeUrl}"/>
+        <meta itemprop="name" content="View Change"/>
+      </div>
+    </div>
+  {/if}
 {/template}