add form for taking snapshot in /danger/snapshots.html template.

The back-end is not implemented at time of commit.

templates/danger/snapshots.html

@@ -18,6 +18,21 @@
         Snapshots go with it. I recommend you download the Snapshots at regular
         intervals and use those as the site's <em>back-ups</em>.
     </p>
+    <form class="be-gui-form"
+              action="/danger/take-snapshot"
+              method="post">
+            <input required
+                   type="hidden"
+                   name="AUTHENTICITY-TOKEN"
+                   value="{{token}}">
+            <button class="be-gui-button danger"
+		    title="Take a snapshot of the website's data and database"
+		    type="submit">
+	        <img alt="Save nav. menu setting for{{db-data.title}}"
+		     src="/images/icons/take-snapshot.png">
+                Take Snapshot
+	    </button>
+    </form>
 
 </main>
 {% endblock %}