Design tweaks to dialogs

Little bit of a mix of things in this one:
 * Support variable-width dialogs. Default is fixed-width as before,
   only UploadConformDialog is variable-width. Controlled by a prop
   to BaseDialog.
 * Fixes to the cancel 'x' - scale the mask image, tweak size & colour
 * Colour & boldness of dialog titles
 * Align the dialog title & cancel 'x'
 * Remove gap between dialog buttons & right hand side of dialog(!)
 * Round corners on dialogs
 * Add grey border on image preview in upload confirm dialog
 * and, squeezing in slightly randomly, finish the partially renamed
   ChatInviteDialog to AddressPickerDialog.

src/Modal.js

@@ -20,6 +20,7 @@ limitations under the License.
 const React = require('react');
 const ReactDOM = require('react-dom');
 import PropTypes from 'prop-types';
+import classNames from 'classnames';
 import Analytics from './Analytics';
 import sdk from './index';
 import dis from './dispatcher';
@@ -158,7 +159,7 @@ class ModalManager {
     }
 
     createDialog(Element, ...rest) {
-        return this.createDialogAsync(new Promise(resolve => resolve(Element)), ...rest);
+        return this.createDialogAsync(Promise.resolve(Element), ...rest);
     }
 
     createTrackedDialogAsync(analyticsAction, analyticsInfo, ...rest) {