Add a form field component for the redesign

The label moves into the border on focus and after being filled. A valid color
is applied to the label and input border. Other states like invalid can be added
later as needed.

Adapted from @ara4n's experiment into a React component with a CSS only
approach.

src/components/views/elements/Field.js

@@ -0,0 +1,51 @@
+/*
+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.
+*/
+
+import React from 'react';
+import PropTypes from 'prop-types';
+
+export default class Field extends React.PureComponent {
+    static propTypes = {
+        // The field's id, which binds the input and label together.
+        id: PropTypes.string.isRequired,
+        // The field's <input> type. Defaults to "text".
+        type: PropTypes.string,
+        // The field's label string.
+        label: PropTypes.string,
+        // The field's placeholder string.
+        placeholder: PropTypes.string,
+        // All other props pass through to the <input>.
+    }
+
+    render() {
+        const extraProps = Object.assign({}, this.props);
+
+        // Remove explicit props
+        delete extraProps.id;
+        delete extraProps.type;
+        delete extraProps.placeholder;
+        delete extraProps.label;
+
+        return <div className="mx_Field">
+            <input id={this.props.id}
+                type={this.props.type || "text"}
+                placeholder={this.props.placeholder}
+                {...extraProps}
+            />
+            <label htmlFor={this.props.id}>{this.props.label}</label>
+        </div>;
+    }
+}