UI fixes in SessionRestoreErrorDialog

* Make the 'delete my data' button not the default
 * Make it red
 * Give it a confirmation dialog
 * Remove the 'cancel' button: what does it mean to cancel an error?
   In this case, it tried again and almost certainly got the same error.
 * Remove the top-right 'x' and don't cancel on esc for the same reason.
 * Move 'send bug report' to a button rather than a 'click here' link
 * Add a 'refresh' button which, even if it's no more likely to work,
   will at least look like it's doing something (it's mostly so if you
   don't have a bug report endpoint, there's still a button other
   than the one that deletes all your data).

src/components/views/elements/DialogButtons.js

@@ -1,5 +1,6 @@
 /*
 Copyright 2017 Aidan Gauland
+Copyright 2018 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.
@@ -14,8 +15,6 @@ See the License for the specific language governing permissions and
 limitations under the License.
 */
 
-"use strict";
-
 import React from "react";
 import PropTypes from "prop-types";
 import { _t } from '../../../languageHandler';
@@ -33,12 +32,21 @@ module.exports = React.createClass({
         // onClick handler for the primary button.
         onPrimaryButtonClick: PropTypes.func.isRequired,
 
+        // should there be a cancel button? default: true
+        hasCancel: PropTypes.bool,
+
         // onClick handler for the cancel button.
-        onCancel: PropTypes.func.isRequired,
+        onCancel: PropTypes.func,
 
         focus: PropTypes.bool,
     },
 
+    getDefaultProps: function() {
+        return {
+            hasCancel: true,
+        }
+    },
+
     _onCancelClick: function() {
         this.props.onCancel();
     },
@@ -57,9 +65,9 @@ module.exports = React.createClass({
                     { this.props.primaryButton }
                 </button>
                 { this.props.children }
-                <button onClick={this._onCancelClick}>
+                { this.props.hasCancel ? <button onClick={this._onCancelClick}>
                     { _t("Cancel") }
-                </button>
+                </button> : null }
             </div>
         );
     },