Remove members field from ProjectEntity

[ablaszkiewicz]

Acceptance criteria

• members field is removed from ProjectEntity, normalized and serialized,
• any method which was referencing that field and all next mehods which were referencing that method are also removed - please do sanity check here and double check if it is really safe to remove Xd,
• tests are adjusted if needed,
• migration to clear members field is added (and applied),
• migration related to index on this field is removed,
• index is removed from the database