Add separate component for post-auth security flows

Instead of twisting `AuthBody`, this adds a new component for the different
styling of post-auth security flows. This also makes them fixed width and
adjusts padding to match designs.

src/components/views/auth/AuthBody.js

@@ -17,29 +17,10 @@ limitations under the License.
 'use strict';
 
 import React from 'react';
-import PropTypes from 'prop-types';
-import classnames from 'classnames';
 
 export default class AuthBody extends React.PureComponent {
-    static PropTypes = {
-        header: PropTypes.bool,
-    };
-
-    static defaultProps = {
-        header: true,
-    };
-
     render() {
-        const classes = {
-            'mx_AuthBody': true,
-            'mx_AuthBody_noHeader': !this.props.header,
-            // XXX The login pages all use a smaller fonts size but we don't want this
-            // for subsequent auth screens like the e2e setup. Doing this a terrible way
-            // for now.
-            'mx_AuthBody_loginRegister': this.props.header,
-        };
-
-        return <div className={classnames(classes)}>
+        return <div className="mx_AuthBody">
             { this.props.children }
         </div>;
     }