add period templates, add period, edit period, delete period

templates/base.html

@@ -24,6 +24,9 @@
             <li class="nav-item">
                 <a class="nav-link" href="/tasks">Tasks</a>
             </li>
+            <li class="nav-item">
+                <a class="nav-link" href="/periods">Periods</a>
+            </li>
             </ul>
         </div>
     </nav>