Add macro for submit button

app/templates/main/index.html.j2

@@ -123,7 +123,7 @@
                 </div>
               </div>
               <div class="card-action right-align">
-                <button class="btn waves-effect waves-light" id="login-form-submit" name="login-form-submit" type="submit">Log in<i class="material-icons right">send</i></button>
+                {{ submit_button(login_form.submit) }}
               </div>
             </form>
           </div>