Add a ToggleSwitch and use it for SettingsFlag

Also bring in the compact timeline option.

Without minor CSS changes, the old user settings are completely unusable with this change. As such, minimal effort has been put in to have it be useful. Similarly, the changes drop the use of radio groups and the old theme selector was the only one that used it. See the comments for more details on how/why this was mitigated the way it was.

src/components/views/elements/ToggleSwitch.js

@@ -0,0 +1,66 @@
+/*
+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';
+import classNames from "classnames";
+
+export default class ToggleSwitch extends React.Component {
+    static propTypes = {
+        // Whether or not this toggle is in the 'on' position. Default false (off).
+        checked: PropTypes.bool,
+
+        // Whether or not the user can interact with the switch
+        disabled: PropTypes.bool,
+
+        // Called when the checked state changes. First argument will be the new state.
+        onChange: PropTypes.func,
+    };
+
+    constructor(props) {
+        super(props);
+
+        this.state = {
+            checked: props.checked || false, // default false
+        };
+    }
+
+    _onClick = (e) => {
+        e.stopPropagation();
+        e.preventDefault();
+
+        if (this.props.disabled) return;
+
+        const newState = !this.state.checked;
+        this.setState({checked: newState});
+        if (this.props.onChange) {
+            this.props.onChange(newState);
+        }
+    };
+
+    render() {
+        const classes = classNames({
+            "mx_ToggleSwitch": true,
+            "mx_ToggleSwitch_on": this.state.checked,
+            "mx_ToggleSwitch_enabled": !this.props.disabled,
+        });
+        return (
+            <div className={classes} onClick={this._onClick}>
+                <div className="mx_ToggleSwitch_ball" />
+            </div>
+        )
+    }
+}