add DeleteGoup function

Co-Authored-By: Talha Caliskan <killerber4t@users.noreply.github.com>

src/main/resources/templates/detailsMember.html

@@ -59,6 +59,12 @@
                                 type="submit">Gruppe verlassen
                         </button>
                     </form>
+                    <form action="/gruppen2/deleteGroup" method="post">
+                        <button class="btn btn-danger" style="border-style: none; margin: 5px"
+                                th:name="group_id" th:value="${group.getId()}"
+                                type="submit">Gruppe löschen
+                        </button>
+                    </form>
                 </div>
             </div>
             <div class="col-3" style="white-space: nowrap">