Use Modernizr to check for browser compatibility

Add a CompatibilityPage which is shown for incompatible clients. If they
continue on regardless, proceed as if it never happened.

src/skins/vector/css/pages/CompatibilityPage.css

@@ -0,0 +1,19 @@
+.mx_CompatibilityPage {
+    width: 100%;
+    height: 100%;
+    background-color: #e55;
+}
+
+.mx_CompatibilityPage_box {
+    position: absolute;
+    top: 0;
+    bottom: 0;
+    left: 0;
+    right: 0;
+    margin: auto;
+    width: 500px;
+    height: 300px;
+    border: 1px solid;
+    padding: 10px;
+    background-color: #fcc;
+}

src/skins/vector/views/pages/CompatibilityPage.js

@@ -0,0 +1,58 @@
+/*
+Copyright 2015 OpenMarket 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.
+*/
+
+'use strict';
+
+var React = require('react');
+
+module.exports = React.createClass({
+    displayName: 'CompatibilityPage',
+    propTypes: {
+        onAccept: React.PropTypes.func
+    },
+
+    getDefaultProps: function() {
+        return {
+            onAccept: function() {} // NOP
+        };
+    },
+
+    onAccept: function() {
+        this.props.onAccept();
+    },
+
+    render: function() {
+
+        return (
+        <div className="mx_CompatibilityPage">
+            <div className="mx_CompatibilityPage_box">
+                <p>Sorry, your browser is <b>not</b> able to run Vector.</p>
+                <p>
+                Buttons and images may appear out of place, communication may
+                not be possible and all manner of chaos may be unleashed.
+                </p>
+                <p>
+                Though if you like taking risks with your life, you can still try it
+                out by clicking that you understand the risks involved.
+                </p>
+                <button onClick={this.onAccept}>
+                    I understand the risks and wish to continue
+                </button>
+            </div>
+        </div>
+        );
+    }
+});