feat: Add Activity export

See #27
See #29
Resolves #24

export/templates/export/export.html

@@ -36,6 +36,15 @@
                        href="{% url 'export:relationship' %}">Export</a>
                 </td>
             </tr>
+
+            <tr>
+                <td>Activities</td>
+                <td></td>
+                <td>
+                    <a class="btn btn-info"
+                       href="{% url 'export:activity' %}">Export</a>
+                </td>
+            </tr>
         </tbody>
     </table>