Simplify the avatar settings and options

This is a bit huge for a micro, but considering how messy the situation is it could be worth.

Some testing would be nice as well! ;D

Basically what is described here:
http://www.elkarte.net/community/index.php?topic=1906.0
* no more permissions for each single method of setting an avatar, just one global permission and each method has an enable/disable flag.
* only one setting for size that can allow the admin to: a) refuse the avatar, b) resize it via css (it attaches a classes and outputs a style tag, so it should be easy to override if needed), c) resize it via php (GD/ImageMagick)
should be everything.