issue2550966: fix suboptimal links in docs. I think I got the majority of them. Some links still don't resolve because they are gone and there is no replacement afaict. Also the check is picking up example links like: https://.../rest and http://localhost:8017/... which should not be checked but are.

def audit_html_files(db, cl, nodeid, newvalues):
    if 'type' in newvalues and newvalues['type'] == 'text/html':
        newvalues['type'] = 'text/plain'
    

def init(db):
    db.file.audit('set', audit_html_files)
    db.file.audit('create', audit_html_files)