actually, move signout warning to UserSettings.js

also, kill off the inexplicably useless LogoutPrompt in favour of a normal QuestionDialog.
This in turn fixes https://github.com/vector-im/riot-web/issues/2152

src/components/views/dialogs/LogoutPrompt.js

@@ -1,61 +0,0 @@
-/*
-Copyright 2015, 2016 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.
-*/
-var React = require('react');
-var dis = require("../../../dispatcher");
-
-module.exports = React.createClass({
-    displayName: 'LogoutPrompt',
-
-    propTypes: {
-        onFinished: React.PropTypes.func,
-    },
-
-    logOut: function() {
-        dis.dispatch({action: 'logout'});
-        if (this.props.onFinished) {
-            this.props.onFinished();
-        }
-    },
-
-    cancelPrompt: function() {
-        if (this.props.onFinished) {
-            this.props.onFinished();
-        }
-    },
-
-    onKeyDown: function(e) {
-        if (e.keyCode === 27) { // escape
-            e.stopPropagation();
-            e.preventDefault();
-            this.cancelPrompt();
-        }
-    },
-
-    render: function() {
-        return (
-            <div>
-                <div className="mx_Dialog_content">
-                    Sign out?
-                </div>
-                <div className="mx_Dialog_buttons" onKeyDown={ this.onKeyDown }>
-                    <button className="mx_Dialog_primary" autoFocus onClick={this.logOut}>Sign Out</button>
-                    <button onClick={this.cancelPrompt}>Cancel</button>
-                </div>
-            </div>
-        );
-    }
-});
-