fixes and cleaning

not  much, it's a mostly a "workaround" for #1036 and a little bit of code cleaning.

The issue is, that the $settings arrray isn't initialized that early in the code and therefore it's impossible to use $settings['images_url'] for the default avatar.

Another possible fix would be to use a $modSettings['default_avatar'] but that would prevent themers from using different default avatars for their theme.

Note: It's working fine on all other areas (display template, profile, pm template etc), it's just the menu without the "default avatar"..