An active surveillance, post-authorization safety study (PASS) of serious infection, malignancy, cardiovascular and other safety events of interest among patients treated with tofacitinib for moderately to severely active rheumatoid arthritis within the British Society for Rheumatology Biologics Register-Rheumatoid Arthritis (BSRBR-RA) (Safety of tofacitinib in BSRBR-RA)