Clean up schema for table environments
From https://gitlab.com/gitlab-com/infrastructure/issues/1709 (private to GitLab employees):
- Wrong data type: created_at should be timestamptz
- Wrong data type: updated_at should be timestamptz
- Missing constraint: created_at NOT NULL
- Missing constraint: updated_at NOT NULL
- Missing constraint: project_id NOT NULL
- Missing FK: project_id -> projects
- Duplicate data: state, environment_type, name vs. slug?