Fix CSS for IE11 (see #51)

Description
-----------

While testing contao/core#8864 I noticed that the back end login of Contao 4.5 and 4.6 looks like this in IE 11:

![ie11screenshot](https://user-images.githubusercontent.com/4970961/45218806-1f59dc00-b2a9-11e8-8980-8ea4ba74d291.png)

This is because Contao 4.5 started using the `<main>` element for the `#main` container. However, IE 11 treats this as an _inline_ element by default.

See also necolas/normalize.css#727

I think this should probably also be added to the `reset.css` of the Contao CSS framework?
https://github.com/contao-components/contao/blob/8.0.2/css/reset.css#L55-L57
```diff
-header,footer,nav,section,aside,article,figure,figcaption {
+header,footer,nav,section,aside,article,figure,figcaption,main {
 	display:block;
 }
```

Commits
-------

83e01d11 fix CSS for IE11
6b9f7aa5 Merge branch '4.6' into ie-fix

src/Resources/contao/themes/flexible/src/basic.css

@@ -13,6 +13,9 @@ html {
 }
 
 /* General */
+header,footer,nav,section,aside,main,article,figure,figcaption {
+	display:block;
+}
 body,button,ul,ol,li,h1,h2,h3,h4,h5,h6,pre,figure,form,fieldset,legend,p,blockquote,table {
 	margin:0;
 	padding:0;