Improved logs, messages and inactive use

Renamed 'Everyone' to 'All Users'

Renamed 'Experimenters' to 'Users'

Logs on email sent/failed as info/error

Rejects submission if an inactive user is explicitly addressed

Disable/Enable Users and Group fields when 'All Users' is
checked/unchecked

Print success message on emailing and how many were sent

Print failure message for any emails that failed to sent

Recoded some of the email resolution code to allow it to be called more
easily during form cleaning rather than email sending