Log requests

In order to get specific statistics on what worked and what didn't (say, for example, to run design or content A-B tests), adhocracy should be able to log all requests in its database, provided the option is turned on using the `adhocracy.requestlog_active` option.

Since IP addresses may allow administrators to link anonymous user accounts to real-life identities, the option `adhocracy.requestlog_ipanonymization` can be used to not log IP addresses at all (default), set the last bits to zero, or log them in full(for example when the identity is given out anyways).

Currently, there is no interface to extract the data. It's not strictly necessary, since this functionality will always be used by system administrators, but we may consider adding one later.