Clean up schema for table ldap_group_links

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: (provider, cn) UNIQUE
• Missing FK: group_id -> namespace