Adds new feature to dispatch avatar url between different servers.

Adds new feature to dispatch avatar url between different servers.
That allows parallel download from most browsers.
To avoid caching issue, server choice is "profile_id" based so that
avatar from a specific user will always be served from same server.

Introduce new configuration parameter:
$config['avatar']['server_modulo'] = 5;

Very easy to implement using, for example, DNS wildcard.
For example, if you have following configuration:
$config['avatar']['server']        = 'static_rrdns_.domain.tld';
$config['avatar']['server_modulo'] = 5;

Then, when building avatar's URL, domain will become, depending on profile_id:
static0.domain.tld
static1.domain.tld
static2.domain.tld
static3.domain.tld
static4.domain.tld