[feature/avatars] Refactor Avatars to support custom modules

This PR contains the changes for adding a modularized avatar system.
Tracker: http://tracker.phpbb.com/browse/PHPBB3-10018
RFC: http://area51.phpbb.com/phpBB/viewtopic.php?f=108&t=32835

It contains the initial commits by Cullen and some more changes by igorw.

Although I have merged those changes into develop just a few days ago it'll likely not merge without issues. I'll try to fix that as soon as possible though.

This system now works with service containers instead of the extension finder.
Changes I can think of right now:

* It actually works now
* Added Gravatar (http://tracker.phpbb.com/browse/PHPBB3-9498)
* Works in the ACP users and group settings
* Use service containers
* Merged on top of develop (around 6 days ago)
* Fixed incorrect coding and W3C validation issues
* Added subsilver2 support
* Added missing doc blocks (probably still needs some work)
* Added avatars to ucp groups page
* Remove obsolete functions for handling avatars in functions_user.php

What still needs to be done:

* Fix any issues you guys mention