cgi.py, operation.py, list.fhtml: Request-level policy switch.

  * Introduce a new configuration variable `ALLOWOP' with a policy flag
    for each request type;

  * have `BaseRequest.check' ensure that the corresponding policy flag
    is set;

  * export this policy switch to the template language; and

  * only show widgets for the permitted operations in the web interface.

The commands still appear in the userv/SSH interface, which is a bit
gnarly.