Provide better feedback on send_invite failures

[2ynn]

Fixes #404

• add reference name + log fields to model
• improve naming for invite_sent -> all_sent field
• show local time instead of UTC
• log successes and errors in onboarding model
• add check if a user is added to list when all_sent is already true

This PR also replaces the deprecated Auth.objects.make_random_password() function