[ADDED] Admin Pool Newsletter

* Added throttle option to swiftmailer for SMTP limiting
* Added newsletter page for admin panel
* Added newsletter mail template (simple one for now)
* Added new global configuration option
* Added default notification setting system in notification class
* Added global pool newsletter enable/disable switch in admin settings

Fixes #2087 once merged.
[FIX] Skip locked accounts
[FIX] Logic error for notification setting
[FIX] Return defaults with no settings at all
Merge pull request #2088 from MPOS/pool-newsletter

Pool newsletter