Mail enabled distribution groups

Summary:
CLI tools for group management

TODO:

• Model Tests,
• Jobs tests,
• LDAP backend and tests,
• Checking Group::emailExists() when creating a user/alias
• Suspending a group when suspending a user (idea abandoned for now)
• Removing a group member when removing a user?
• Deleting a user should delete groups
• force-deleting a group should remove entitlement records
• Limit "owner" of a "group" to be the same owner as the domain in which the group is to exist (could be automatic).
  • Limits groups to custom (non-public) domains.

Test Plan: ./phpunit

Reviewers: #hkccp_developers, vanmeeuwen

Reviewed By: #hkccp_developers, vanmeeuwen

Subscribers: vanmeeuwen, #hkccp_developers

Differential Revision: https://git.kolab.org/D2020