[WIP][ticket/12662] Reorganize and modernize the header navbar

https://tracker.phpbb.com/browse/PHPBB3-12662
http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=45679&start=60

![responsive](https://cloud.githubusercontent.com/assets/3015760/3204191/a84a6142-eda5-11e3-900e-91e43b666c4f.png)

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![loggedout](https://cloud.githubusercontent.com/assets/3015760/3208288/085885d2-ee28-11e3-89f8-f733f0dbb24e.png)

PHPBB3-12662
agreed `.header-avatar:after { content: "U+25BC"; }`