A Sandwich ELISA for Detection of Soluble GPI‐80, a Glycosylphosphatidyl‐Inositol (GPI)‐Anchored Protein on Human Leukocytes Involved in Regulation of Neutrophil Adherence and Migration—Its Release from Activated Neutrophils and Presence in Synovial Fluid of Rheumatoid Arthritis Patients—