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/components/views/dialogs/UploadConfirmDialog.js

@@ -87,6 +87,7 @@ export default class UploadConfirmDialog extends React.Component {
 
         return (
             <BaseDialog className='mx_UploadConfirmDialog'
+                fixedWidth={false}
                 onFinished={this._onCancelClick}
                 title={title}
                 contentId='mx_Dialog_content'