Allow checksum expire to be configured

Currently the number of days a checksum will last is hard-coded in CRM/Contact/BAO/Contact/Utils.php, as the number 7. This would be extremely easy to replace with a PHP constant, or a CRM_CORE_CONFIG setting. I don't know enough about civi architecture to know the correct approach, but here's an idea:
Add this to civicrm_settings.php:

/**

• This is the default number of days before a checksum (hashed) link from CiviMail will expire.
  */
  define( 'CIVICRM_CS_TIMEOUT' , 7);

And then in CRM/Contact/BAO/Contact/Utils.php, change this:
$live = 24 * 7;
to this:
$live = 24 * CIVICRM_CS_TIMEOUT;