Clean up schema for table pages_domains
From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):
- Wrong data type: certificate could be bytea
- Wrong data type: encrypted_key could be bytea
- Wrong data type: encrypted_key_iv could be bytea
- Wrong data type: encrypted_key_salt could be bytea
- Missing constraint: domain NOT NULL
- Missing constraint: project_id NOT NULL
- Missing FK: project_id -> projects