Add setting to limit number of same e-mails

[ywarnier]

Chamilo installations allow by default to use several times the same e-mail for different users creation.
This leads to abuses, for example on campus.chamilo.org, with some people creating more than 1500 users with the same e-mail address.

Add a configuration setting:

// Set a maximum number of identical e-mail. Set to 0 to remove the limit.
$_configuration['hosting_limit_identical_email'] = 0;

...and modify the inscription process to take this limit into account.
$maxIdenticalMailsReached = "The e-mail address %s has already been used by the maximum configured limit of accounts. Please use another e-mail address.";