Fix h2 in markdown being weird

Date separator style was just define don any h2 in the message
list, so got picked up by any h2s in markdown too. Give date
separators their own class.

Fixes https://github.com/vector-im/riot-web/issues/4328

src/components/views/messages/DateSeparator.js

@@ -53,7 +53,7 @@ module.exports = React.createClass({
         }
 
         return (
-            <h2>{ label }</h2>
+            <h2 className="mx_DateSeparator">{ label }</h2>
         );
     }
 });