Extract System emails from enotify

- Removed every SYSTEM_EMAIL occurrence in enotify
- Introduced a "SystemMailBuilder" for build system emails
- Replaced every SYSTEM_EMAIL usage in the classes with calling this builder
- Added tests for this new Builder
- Split the email templates between "base" template for email and concrete usages for different use cases

boot.php

@@ -152,8 +152,6 @@ define('NOTIFY_TAGSHARE',   256);
 define('NOTIFY_POKE',       512);
 define('NOTIFY_SHARE',     1024);
 
-define('SYSTEM_EMAIL',    16384);
-
 define('NOTIFY_SYSTEM',   32768);
 /* @}*/