Simple mail : Make max emails a hidden config instead of hardcoded value set to 50Si

Allow advanced users to alter the limit for sending emails from search.

Currently , sending direct email to more than one user from tasks in contact search page is limited and hardcoded to only 50 contacts. This is very inflexible for some use cases (e.g donor management teams might have a need for more frequent interaction with their contacts without wanting it to appear as a civimail).

However, there is real risk to organisations, and the reputation of CiviCRM, if we make it so easy to alter this limit that people do it without understanding the implications. Specifically if they break spam laws they may open themselves up to hefty fines, or they could get their domain (& possibly some markers of CiviCRM emails) identified as spam. A compromise is to recognise that it is already possible to override the limit by users who have file access and to make that override ability more maintainable (by putting it in a setting called simple_mail_limit) which can be set using the api or through a setting override, but not to expose it in the UI where inexpert users might find it. 

Note this is settable via api explorer and hosting providers who wish to block that (ping Hershel Robinson) should use a setting override in the civicrm.settings file e.g

global $civicrm_setting;

$civicrm_setting['simple_mail_limit'] = 50;