Various improvements

 * filter actions by name not constant [from vaal]
 * better robustness against failure to generate an admin link for an object [from tgandor]
 * better performance by prefetching content types [from rwmcfa1]
 * filter by staff users only [from gotlium, updated for custom user models]
 * don't allow delete action for LogEntry
 * tidy up admin list, including linking users and objects
 * tidy up display of each individual LogEntry to make things clearer