issue2551099 - disable processing of data url's in markdown. Display as plain text. Added 'data' to templating.py _disable_url_schemes array. User should be able to re-enable it by changing the array from the tracker's interfaces.py. Markdown tests failed before the change to _disable_url_schemes. Also add second separate data test for ReST and plain text processing. data url's look like they are already ignored in these proess streams.

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)