Add validation feedback helper

This adds a general validation feedback mechanism for checking input values. An
initial example is wired up for the username input on registration.

res/css/views/elements/_Validation.scss

@@ -0,0 +1,32 @@
+/*
+Copyright 2019 New Vector Ltd
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+.mx_Validation_details {
+    padding-left: 15px;
+}
+
+.mx_Validation_detail {
+    font-weight: normal;
+
+    // TODO: Check / cross images
+    &.mx_Validation_valid {
+        color: $input-valid-border-color;
+    }
+
+    &.mx_Validation_invalid {
+        color: $input-invalid-border-color;
+    }
+}