digest: Display the text email also in the /digest view.

templates/zerver/digest_base.html

@@ -18,6 +18,7 @@
                 {% include 'zerver/emails/compiled/digest.html' %}
                 <img id="digest-footer" src="/static/images/emails/footer.png"/>
             </div>
+            <pre>{% include 'zerver/emails/digest.txt' %}</pre>
             <div class="digest-address-link"> {{physical_address}}</div>
         </div>
         {% include 'zerver/footer.html' %}