bg-card will be added to the class and password/username validation

bg-card should not be an id, ids should be unique in their use.

src/main/resources/templates/error.html

@@ -10,7 +10,7 @@
         <th:block th:insert="~{fragments/navbar.html :: navbar}"></th:block>
         <div class="container">
           <div th:insert="~{fragments/errorBanner.html :: errorBanner}"></div>
-          <div id="bg-card" class="text-center">
+          <div class="bg-card text-center">
             <h1 class="display-2" th:text="#{oops}"></h1>
             <p class="lead" th:if="${param.status == '404'}" th:text="#{error.404.1}"></p>
             <p class="lead" th:unless="${param.status == '404'}" th:text="#{error.404.2}"></p>