[ticket/12864] Add template event to header after stylesheets

https://tracker.phpbb.com/browse/PHPBB3-12864
Allows users to place simple stuff after the included CSS files, in case an ext author needed to simply overwrite some CSS rules, e.g.:
```css
<style>
  body { font-size: 1em; }
  p { color: orange; }
</style>
```
Reduces the need for ext authors to use !important as we currently must do. It's also useful for other reasons too :smile: 

PHPBB3-12864
Should have a comment to explizit state that INCLUDECSS will not work here anymore