Exclude non-existing users from group member list iteration, ensure cascade deletion from respective models

Summary:

• Avoid an undefined method email' for nil:NilClass` (for existing records)
• Configure dependent action on user and group models, to avoid leaving orphan membership records

Test Plan: webdns4.grnet.gr

Reviewers: #servers, stefanosliv

Reviewed By: #servers, stefanosliv

Subscribers: #servers

Differential Revision: https://repo.grnet.gr/D62