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 init_severity(db, cl, nodeid, newvalues):
    """Make sure severity is set on new issues"""
    if 'severity' in newvalues and newvalues['severity']:
        return

    normal = db.severity.lookup('normal')
    newvalues['severity'] = normal

def init(db):
    db.issue.audit('create', init_severity)