Invitations can now be configured through admin panel settings. By
default, invitations are enabled. Invitation system is also available if
registrations are disabled. To completely remove the ability of new
users to sign up, disable both registration and invitations.
Fixes#330
This will allow users to send invitations to other people via email.
Each account will still need to confirm the email address if the option
is enabled.
Addresses #330, will need to allow pool operators to enable this feature
even with registrations turned off.