Use h1 as first heading in dialogs (#12250)

* Use h1 as first heading in dialogs

The dialog should be modal and therefore the only content active
on the screen, so the title of the dialog should be the top level.

Update snapshots & tests appropriately.

* Just customise the letter spacing on dialog titles

src/components/views/dialogs/BaseDialog.tsx

@@ -179,7 +179,7 @@ export default class BaseDialog extends React.Component<IProps> {
                         {!!(this.props.title || headerImage) && (
                             <Heading
                                 size="3"
-                                as="h2"
+                                as="h1"
                                 className={classNames("mx_Dialog_title", this.props.titleClass)}
                                 id="mx_BaseDialog_title"
                             >