Clean up schema for table project_group_links

Per 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 FK: project_id -> projects
• Missing FK: group_id -> namespaces (2 entries violate this rule)
• Missing constraint: UNIQUE (project_id, group_id)